If your heater blows cold air, your engine is getting up to temp, and all your switches and actuators are working properly this is your problem....
For reference this is being performed on a 2000 S10 Blazer 2dr 4.3vin W w/AC
Basically GM uses dexcool, when neglected or after becoming too diluted with water over a long period of time it likes to turn into a nice brown sludge of a substance that clogs up your heater core and everything else in your cooling system. Im going to show you how to flush the heater core out with some simple items you can buy at your local parts store.
-5/8in. Garden hose I used a 50 footer that they had at O'Riely's for $9.99
-Adapter's for your heater hoses (on mine one side was 5/8 already and the other was 3/4 so I bought a 5/8-3/4 adapter) $2.99
-Section of hose to mate the adapter to the heater core (in my case 1 ft. 3/4 heater hose) $1.99
-Optional screw type hose clamps (I bought them and didnt use them alot of people dont line the spring type clamps I have a special set of pliers that takes them off so I like them, up to you) $1.99
Heres a picture of everything I used.
Next you'll remove the two hoses from the firewall that go into the heater core shown above.
Then youll attach the end of the garden hose attached to the spicket onto the drivers side heater core tube. Of course the adapter, 3/4 hose, and other length of garden hose which should go into some container an old garbage can or something to catch the mess thats soon to flow out of that end of the hose. Then you turn the spicket on really low at first until you fill the heater core up and water starts to drip out of the end of the drain hose. Once you have a steady flow coming out of the hose turn it up a bit at a time until your at good flowing flush rate that your comfortable with I didnt go too strong at fear of over pressurizing the heater core and blowing it up all over my passenger side floormat. When I was done I reversed the garden hose ends and flushed it in reverse or backflushed it just for good measure.
Thats it my truck is nothing short of hot inside in washington state 28 degree weather and it did not work one bit before this, I could swear the A/C was warmer prior to flushing.
There is a slightly cheaper and easier way to do this in some events you wont need anything other than a set of pliers... If you own a truck where the heater hoses are both the same size or close enough to it that you can just swap the hoses from side to side thats all you have to do alot of times to get it unclogged. However keep in mind that if you try to stretch an older 5/8 hose onto a 3/4 port there is a good chance your gonna rip it, be prepared to buy a new hose. Also doing it that way just flushes that dexcool mud right back into your system... I prefer the way I did it so I can see the mud hit the ground and leave my truck for good.
For Best Results:
Do this at the same time you are flushing your cooling system. Drain the coolant take the radiator and heater hoses off, flush the block, radiator, and heater core seperately using pretty much the same method shown above and refill the whole system with new coolant im going to tell you to replace dexcool with dexcool because thats what I do and thats what chevy would want you to do. Ive been a mechanic for many years ive seen many people swap to regular green coolant. I do know the active chemical in the two are the same dexcool is 80-95 percent ethylene glycol (the stuff green antifreeze is made out of) it does however have no silicates which are harmful to seals in the coolant passages and waterpump also has no nitrite, borate, phosphate, nitrate and amine, which elude my knowledge but they arent there lol. The myth that the water pump will go out because dexcool is not there to "lube" it is not true, water pump bearings are sealed and upon entry of coolant to the bearings the pump will fail. So use green at your own risk test have proven that dexcool in its prime state will make your pump last longer as its chemical make up is not as harsh on the seals and has been proven to maintain nearly 100 percent of its concentration after 150k miles while green is 50k or so, but other than that I see no reason that you cant use green.
Enjoy the heat, hope this helps good luck.