1991 735 stock radio not working - The BMW 7 Series (E32) Forum

Alright, so I'm generally an e28 guy, but a buddy brought over his e32 yesterday (along with a case of beer) and asked me if I could help him work on it. Needless to say the beer lured me in.

Anyhow I've searched this forum multiple times and gone through countless posts trying to find a similar problem, but I must not be searching the correct terms. Sorry for making a new post, I'm sure this has been covered.

So my friend took his 91 e32 to our local bimmer mechanic (Jim at Der Meister in Tallahassee, if anyone needs a good mechanic) with radio problems and he said the amplifier was bad. The car would not play anything. So my friend replaced the amplifier with an ebay one, and the radio worked normally for a couple days, then cut out and will only play static.

So I did all the normal stuff, I replaced the fuse and checked the wiring bundles for chafing or bad wires. I couldn't find the ground to check/clean. Although my father told me all the stereo shares a common ground? Does anyone know where that would be?

I pulled the head-unit and looked for the same problems. I also found a wiring schematic and was going to jumper a speaker and bypass the amplifier but the schematic must have shown a different model head unit, because I could only locate the outputs, not the inputs. If anyone could tell me which ones to jumper to bypass the amp, in order to check the head-unit, or send me in right direction for a correct wiring schematic that would be appreciated.

Also is there a way to check if the amplifier is good? I imagine I can check power in/power out with a multi-meter, but is there anything else? If anyone can help point me in the right direction towards a stock stereo troubleshooting guide that would be extremely helpful. Sorry about the long post, thanks in advance for any replies!