1990 Audi 90 stereo help please!

So you did have the amplified system. You cant tap into any of that wiring. You must run a pair of speaker wires from the radio to the back of your car and install a new pair of speakers.  Why use those factory speakers, they have no bass, they suck!  (you knew that).  The factory wiring that goes back there has a common ground, and its just best to put in some new wires. I find that on the audis the easiest thing to do is to run the wires along the tops of the doors, you can tuck in under the plastic, and you can run down into the back that way.  The plastic that runs along the windsheild pops off, and you can hide it under there.  Its a pain, but if you want good sound, thats what you have to do. Another wonderful audi problem is alternator whine. make sure you ground your stereo good.  that tiny tiny little ground wire that audi gives you wont cut it.  put a decent wire in and bolt it to a solid hunk of metal.