When these 30 year plus components fail I prefer to do this,
Remove head unit in dash
Remove CCM inside dash under glove box, bad PIA
Remove all 4 speaker units
Send them all to the nearest factory car stereo Bose repair place for testing.
It could be one thing, or multiple things.
My guy is Bob.
Factory Car Stereo Repair
https://www.carstereohelp.com/