I believe his problem is he doesn't know what his problem is. Could need to be rebuilt. Or not. Depends if the stereo shop screwed it up when installing the stereo system or maybe they only screwed up his wiring or maybe it was just coincidence that it stopped working while they had it.
He needs to get an 89 factory service manual that has step by step diagnosis charts for each gauge listing the correct procedure to find out what is wrong with each gauge. He also needs to get the installation manual for the particular stereo he has and verify all the wiring was done correctly. Hopefully it was a high end unit with a good install manual. Cheaper ones tend to have pretty poor instructions. Just google the brand and model and "installation instructions"
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Here's the 89 S10 Blazer cluster wiring showing both pinouts:
Open the diagram in a new window or tab and click on it to see it larger and clearer.
I start by turning on the key without starting the engine and unplug the oil pressure sender and dark green temperature sensor one at a time. Then ground each of those wires one at a time while watching the gauges. They both should go from one extreme to the other when open or grounded. This will tell you if the gauges are any good. Could just be bad
senders? Same for fuel, but that's a lot more work to unplug the sender.