1990 Audi 100 stereo installation problem

   That's hilarious! I had the exact same car. German cars suck. The electrical system in German cars is among the worst in the world. I feel your pain. Sorry for laughing, but I was there. The guy that put the radio in my car before I got it, hooked the remote turn on up to the heated seat control. You couldn't get power to the radio until you turned on the heat for the drivers seat. Talk about sucking.... I'm just a regular guy. I don't know why all the fuss about me....

if you don't have a volt meter, at the very least please use a test light to verify those wires that you've found. off the top of my head i think constant +12v should be red or RED / white, ground should be brown, and accessory (in this case it sorta acts like a key sense) should be BROWN / red, illumination is probably grey/blue or grey/red. the front speakers wires can be found in the radio harness, but for the rear speakers you will need to bypass the amps that are on each of the rear speakers. to do so the simple way would be not so easy to explain in words on a forum. i recommend you find a competent installer that could do it for you and save you a whole lot of time. you could trigger the rear speakers to turn on by connecting the remote turn-on from your h/u to the white (thin gauge) wire(s) at the radio harness, but usually that will result in engine noise. i agree that german cars are infamous for being electrically challenging. but with some patience and some careful diagnosis, they're really not all that bad. good luck!