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413 Posts
Discussion Starter · #1 · (Edited)
1996 Sonoma SLS RWD 2.2L MT Reg cab, 70k miles.
Future, full-time daily driver.

Summary. This is a long ramble/rant. Currently raining outside where the relevant truck is. However, if you found this thread through search, the photos might be somewhat helpful if you want to understand drainage on the right, front side of the cab, assuming you do not already. You can skip the text and just view the photos, also.

I found rust in the inside, right, front, cab corner, above and below the electrical "bulkhead" connector. Super disappointed as I had grill, bumper, and inner fender out last year, as well as the carpet removed. Spent at least a couple days cleaning the extremely dirty interior. Did not notice this at the time. Venting a bit, but the photos may help others when they are trying to understand water drainage and the sheet metal structure of the cab in this area. Remember, the electrical bulkhead connector on the right side is relocated to below the fresh air damper in later years.

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Wide-angle view of R, front cab interior. Bottom of fresh air damper assembly is the black plastic with triangles, slightly above center of photo.

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Close-up view of R, front cab interior. Sealer rust-jacked. Fresh air damper is to the left, on the "firewall", a few inches higher. The right, plenum drain is directly above and little left of the yellow, circled area, on the exterior.

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Lower, wide-angle view of R, front cab interior. The sealer applied left and right is at the bottom of the "firewall"

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Lower, close-up view of R, front cab interior.

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Wide-angle photo I found on S10forum showing exterior of the area shown in previous 4-photos. Use the bulkhead connector as a "landmark" to understand relationship between sheet metal on outside and inside. Note portion of blower motor visible above wheel, mounted in the HVAC / evaporator enclosure. With fender in place, the area forward of the door in the original red paint area, is partially blocked with a rigid foam seal loosely fixed in place, until it disintegrates or is chewed up by rodents.

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Close-up photo I found on S10forum showing exterior of the area shown in first 4-photos. The opening above the bulkhead connector is the right drain for the plenum. The fresh air damper assembly is an open-topped box with dampers, directly above it that allows the HVAC control panel to select between recirculation or fresh air. Debris entering the plenum and cowl gutter through or past the cowl trim can flush through the drain; get "sucked" through the blower motor to be deposited on the inlet side of the evaporator; or become lodged in place to rot away your sheet metal and help water find seam/seal failures to enter the interior of the cab.

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Back to my truck. Exterior, wide-angle view of top, rear of right fender. Right, cowl trim piece removed. Just to the right (from perspective of this photo) of the hose nozzle, under the middle cowl trim, the plenum drops down to the fresh air assembly and associated sheet metal enclosure. Super difficult to access, but a small diameter wet/dry vac hose can be snaked down to that area to remove debris. I have not found a way to access it through the blower motor opening; too little room. Then, water flushing can be used to hopefully push the remainder out the drain.

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Closer view of top, rear of right fender. Notice all the painted seam sealer. I believe that seam extends to the bottom of the cab. Also notice the intact foam fender seal. This area does not really drain into the plenum, but straight down, past the drain opening by the bulkhead connector, to the bottom of the fender.

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Very close of view of seam sealer below bottom, right corner of windshield. Dry and cracked, as it is below. Could not get photo of seam sealer below, but could see it poorly by prying foam block back going through the door / fender gap.

~30 minutes with a garden hose applying water in multiple locations from roof above door to middle of cowl trim did not yield any interior leaks. I can see no obvious, old water “trails” on the inside. The carpet on the right side of the cab was damp, not soaked, when I pulled it after purchasing the vehicle July 2020, but the floorboard was very corrosion-free. Carpet and the insulator pad at the bottom of the firewall, right side, have remained dry AFAIK. There was tons of debris behind the blower motor and evaporator; in the door hinge areas; in the back of the fenders; and in most of the cowl gutter / plenum. I spent hours cleaning everything out Fall 2020 and water flows freely since. The right (and left) door, drip weatherstrips were replaced with the windshield a couple months ago.

I suspect the rust we see on the inside is the result of normal degradation of the seam sealer on the exterior, and probably previous accumulation of debris that obstructed drainage and maintained a damp environment. I wonder what the plenum area looks like behind the drain, below the fresh air assembly. I noticed no rot on the front of the firewall in that area when I had the inner fenders out. Do not have the motivation to properly address this right now. I think I might use a nozzle extension to Fluid Film the exterior opposite the rust visible inside to slow the rust until/if I am ready to “fix” whatever is rusted.


Two tones of terror
750 Posts
My truck has the same thing and it's from AZ. The factory seam sealed both sides of the panels and this didn't allow water to escape and it never dries out and you get rust. See it all the time. I'm pretty sure all second gens have this just most people don't realize it.

413 Posts
Discussion Starter · #4 ·
i dont see any rust in your pics?
2nd & 4th photos. This thread is intended primarily as an aid to others, primarily by way of photos. I would like this forum not to die. Traffic, and helpful user posts on auto forums for 90’s & 2000’s vehicles is far below what it was 10 years ago. This thread will not reach maximum usefulness unless/until I remove the fender (and everything else required to do that) and remove seam sealer on the inside and outside to expose the extent of the rust. My guess is the rust started on the outside and worked its way through the seam(s) and/or spot welds, unless water was directly intruding into the interior, which I have thus far been unable to replicate.

My truck has the same thing and it's from AZ. The factory seam sealed both sides of the panels and this didn't allow water to escape and it never dries out and you get rust. See it all the time. I'm pretty sure all second gens have this just most people don't realize it.
Agree with that, or at least with it being super common, but not recognized. I know I get a little water through the fresh air damper assembly on my 02 daily driver, but it is not worth fixing given the condition of the vehicle and difficulty in repair.

413 Posts
Discussion Starter · #6 ·
Update. Removed the R fender to allow for inspection. R cowl drain was full of debris. Now that I can visually inspect the area behind the fender, it is clear that the drain is directly above the worst rust I currently observe inside the cab at the top of what is often called the “kick panel” area. Directly above the drain is the Air (fresh) Inlet Assembly, which is rear of the blower motor. I will likely try to remove the dash and Air (fresh) Inlet Assembly later today. I hope I do not break anything, or at least nothing difficult to replace or repair. Intend to update this thread as I disassemble and get a better view of the rust.

So, why do I care about this? The relevant truck is intended to be my next daily driver. I would like to get 200k more miles out of it, and do not want the cab rotting away until I get closer to that goal.

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Wide angle view of R front cab corner with sheet metal removed.

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Cowl R drain outlet.

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Partially obstructed view of interior cab rust below and opposite the drain outlet. Some seam sealer removed for inspection.

413 Posts
Discussion Starter · #7 · (Edited)
I am using the Factory Service Manual and the following thread to access and remove the Air (fresh) Inlet Assembly.

Front is front. Rear is rear. Left is driver side. Right is passenger side.

Started working on the dash; termed Instrument Panel (I/P) in factory literature. Provided are photos of the speaker covers; center defrost cover; and top of dash. The speaker covers are polycarbonate; likely very brittle; and easy to break. The defrost cover is a 2-piece assembly, with the top, interior-colored portion being metal and the lower, black portion being Polyphenylene Ether (PPE). It is much more durable, but excellent condition replacements are difficult to source. Try not to apply any more pressure to the dash than is necessary.

When removing the defrost cover, some recommend trying to insert a thin blade of metal or alternative beneath the edge of the cover to push on the spring leg of a retaining clip as you pull on that clip with something like a hook-ended pick. I removed the cover 1 – clip at a time, using a hook pick, starting at 1 – side and working my way around the rear till I reached the other side.

The speaker covers are easily removed by first removing the 2 – screws at the rear of each cover using a T15 bit. Then, use the screw threads to grab the rear of the cover and lift up just enough to clear the dash and pull back. There are fingers on the bottom of the cover near the front that hold the front down. If you apply much upward force to the cover before those fingers are slid rear, out of the mating slots, the fingers will break off.

All plastics get cleaned and then treated with 303 Aerospace Protectant. I treat top and bottom sides when accessible in the hope of extending the life of the part.

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Defrost cover removal.

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Defrost vent with cover removed.

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1 - defrost vent mounting clip recess at rear of opening.

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1 - defrost vent mounting clip recess at side of opening.

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Defrost cover removed – top view.

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Defrost cover removed – bottom view.

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Defrost cover mounting clip close-up.

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R speaker cover removal.

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R speaker in top of dash. Note the slots in front for the fingers on the bottom of the cover.

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R & L speaker covers – bottom view.

413 Posts
Discussion Starter · #9 · (Edited)
Disable Supplemental Inflatable Restraint. Disconnect negative battery terminal if battery not already removed. Steering wheel straight w/ no key in ignition. Remove Fuse 22 from Fuse Block (R side of I/P).

Remove L I/P Sound Insulator. 1 – M6-1.50 hex nut (10mm) by Accelerator. 1 – M4.2-1.79 x 14mm 12mm washer SEM screw (7mm) L of Clutch pedal. 2 – M4.2-1.79 x 18mm 12mm washer SEM screws (7mm) for DLC connector. 3 - M4.2-1.79 x 28mm 17mm washer SEM screws (7mm) at top of Insulator. 1 – electrical connector for Interior Lamp Control Module Assembly mounted to “back” side of Insulator.

Remove R Door Sill Trim. Kick Panel Trim. You can pry it straight up. Safer is to compress the individual clips from below the Cab and push each clip up through the floor a bit till the trim pops free.

Remove R Kick Panel Trim. Reference photo to see where you need to pull/pry to unclip it. When unclipped, rotate the bottom down and rearward to un-mate it from the trim piece above it. The Air Bag (SIR) Control Module can be left attached to the Panel. Just disengage the connector lock and disconnect from harness.

Remove R I/P Sound Insulator. 4 - M4.2-1.79 x 23mm 17mm washer SEM screws (7mm).

Remove Center Console (manual transmission). Review photos. Wiggle and pull straight up. Remove 2 - M6-1.0 free-spinning washer hex nuts (10mm) to remove base.

Remove Center I/P Sound Insulator. 1 - M4.2-1.79 x 14mm 12mm washer SEM DST screw (7mm) at L side. 3 - M4.2-1.79 x 19mm 12mm washer SEM screws (7mm) at top. 1 - M4.2-1.79 x 14mm 12mm washer SEM screw (7mm) at bottom R side.

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L I/P Sound Insulator.

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R Door Sill Trim.

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R Kick Panel Trim – back side. Notice location of mounting clip near top, and the post near the bottom which never received a mounting clip.

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R I/P Sound Insulator

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Center Console (manual transmission) and Center I/P Sound Insulator.

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Center Console (manual transmission) – back side mounting clips.

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Center Console (manual transmission) base.

413 Posts
Discussion Starter · #10 ·
Continuing on. Removing the Knee Bolster.

Remove Ash Tray by pressing up on the bright silver, spring metal retainer and pulling the Ash Tray out.

Disconnect Parking Brake Release Handle cable from lever. Remove the slug at end of cable from holder. Also slide cable out of bracket.

Remove Knee Bolster fasteners. 2 – M4.2-1.79 x 18mm 12mm washer SEM screws (7mm) below Steering Column for HVAC duct and courtesy lamp. 2 – M4.2-1.79 x 18mm 12mm washer SEM screws (7mm) at upper, L corner of Ash Tray opening. 2 - M4.2-1.79 x 28mm 17mm washer SEM screws (7mm) at the bottom near where the Center Console was. 4 - M6-1.0 x 25mm flanged button screws (T30).

Lower bolster carefully to avoid damaging HVAC duct and 12V Accessory Port Wires. The Parking Brake Release Handle cable may have to be guided out as it tends to snag on stuff. Remove 4 – individual wires and the 1- connector with the 2 – wires for the main port.

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Knee Bolster.

Gas Machine Auto part Nickel Fixture

Ash Tray retainer.

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Parking Brake Release Handle cable connected.

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Parking Brake Release Handle cable disconnected.

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HVAC duct and courtesy lamp that were connected to bottom of Knee Bolster.

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12V Accessory Port Wires.

2,656 Posts
Your right side speaker cover has the front hooks broken off 😟

For pulling the speaker covers and defrost grille I have a plastic/nylon trim stick that works great for me. The screw trick on the grills is a nice trick.

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413 Posts
Discussion Starter · #15 ·
The holes and the hooks are fragile. I'd say 80% of the covers I find in the yard have one of the two broken.
If I broke them, I likely did the damage 1st time I pulled them when detailing the filthy interior after I bought it 2020. I was gentle this time. Maybe the photos will prevent someone from making the same mistake. I hate removing automotive interior trim components when I do not understand how they are attached.

Premium Member
S10's currently: 96 4wd XCab driver, 98 Blazer winter beater, 95 Jimmy w/LT1, 82 LWB Sport.
123 Posts
One of the great joys in my life is that moment when I realize that I did figure it out, didn't break anything, and don't have more work to do to fix the breakage. In your defense, that plastic, like the defrost vent grille, may have been fragile from the years of sun and heat.

413 Posts
Discussion Starter · #19 · (Edited)
Continuing on.

Removing the L Wheelhouse.

Support Electronic Brake Control (EBC) module if you are concerned about stressing brake lines.

Separate EBC Brackets from Wheelhouse and Fender. Remove 3 - bolts from Brackets. 1 - M8-1.25 x 30mm flanged hex bolt (10mm) at fender and 2 – M8-1.25 x 27mm 17mm washer SEM screws (13mm) from below Wheelhouse for Lower Bracket on Engine side. Upper and Lower EBC Brackets can remain bolted together.

Remove rest of M8-1.5 x 25mm bolts. 3 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) for Reinforcement at front, bottom of Wheelhouse. 2 – M8-1.25 x 22mm 32mm washer SEM screws (13mm) for Firewall tabs. 6 – M8-1.25 x 25mm 24mm washer SEM screws (13mm) for Fender connection.

Wheelhouse will wiggle loose and out. The Windshield Washer Solvent Container is still bolted to the Fender. Lower the Wheelhouse to separate from Container before pulling outwards.

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Supporting EBC module.

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EBC bolt hole just below Fender lip. Bolt already removed.

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Note routing of Hood Latch Release Cable through EBC Lower Bracket legs for reassembly.

413 Posts
Discussion Starter · #20 · (Edited)
Continuing on.

Removing the L Fender.

Windshield Washer Solvent Container and immediate area. Remove 1 - M6.3-1.81 x 20mm 17mm washer SEM screw connecting top of Container to Fender. Support (simpler) in place or remove (more work). Disconnect horn inside fender from wire harness. Open retainer for Hood Latch Release Cable. Open 4 – split wire loom retainers.

Battery (+12Vdc) Junction Block and immediate area. Remove 2 – plastic nuts (13mm) from Junction Block Cover and remove Cover. Remove 2 – M6-1.0 studs and pull Block away from Fender. Open 1 – split wire loom retainer. Free 4 – electrical connectors from push nail (Christmas tree) mounts.

Unbolt Fender. Remove 9 – bolts; all M8-1.25 x 30mm 24mm washer SEM (13mm). 2 – bolts at hood hinge location. 2 – bolts at rear edge of Fender, accessible from Cab area through gap between Door and Cab. 1 – bolt at rear, bottom of Fender. 1 – bolt from back of Radiator Support; somewhat hidden by Washer Solvent Container. 2 – bolts at front, bottom of Fender. 1 – bolt at front, top of Fender. Slip Fender forward, away from door and remove.

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Windshield Washer Solvent Container mounting bolt.

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Battery (+12Vdc) Junction Block.

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View of Battery (+12Vdc) Junction Block area from below.

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L Wheelhouse and Fender Removed.

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Front, L edge of cab.

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L Cowl Drain Outlet. Just below Hood Hinge area. I cleaned this section of the Cowl before I had the windshield replaced last year, but did not notice the Outlet at that time.

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Failing seam sealer in Hood Hinge area just below windshield.

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More crumbling seam sealer at front, L edge of cab.
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