Well, that's not fun.
Got all the aftermarket audio equipment pulled. Plugged in the factory radio and... sounds distorted on the left channel, right channel completely out. Pop sounds when I push the buttons. It worked fine when pulled 32-years ago, so I'm guessing something that ages has gone bad. Probably electrolytic capacitors. I've read there are some green ones that like to go bad and leak.
I just learned today the amp/radio module is the same as used in GMT400 pickups. I guess that explains why these radios are modular and can be taken apart into smaller modules. Vehicles with the split radios used the same modules with cables connecting them.