It sounds to me like the power amplifier is going into oscillation (that's the squeal you hear). The problem with all of these radios is that they are nearing twenty years of age and that's way beyond the lifetime of some parts...under much gentler conditions on average!
After so much freezing and baking, any electrolytic capacitors in the radio are likely toast, and some other components may have drifted out of tolerance. The usual malfunction is a loss of audio, low audio level or loud crackling from the radio only/tape deck units.
At the very least, every electrolytic capacitor on the power amplifier board should be replaced. This won't cost much, provided you can solder. However, GM coated the circuit boards in these radios with some sort of protective substance and I don't know what will take it off. I tried a lot of things while trying to repair the UM6 (AM/FM/cassette) radio from an '88 Buick. I gave up because that coating would not yield.
There are places that repair these radios. M & R electronics is one that I'm considering doing business with, since the stock radio from my Caprice wagon has a clock display that never goes out and a few other issues. They've been around for a long time, as one of my Delco audio service books from 20 years ago mentions their name. I don't know how reputable they are, and have yet to do any business with them. I was quoted $100-150 on the phone to repair all that was wrong with my radio, which did not seem unreasonable since I want to keep the car all stock. It's certainly not as horrifying as I'd have expected.
And if by chance anyone has service manuals for these radios...I'd be very interested! (What I'm looking for would contain schematics, not just troubleshooting.)
