I bought a JVC head unit for my 1990 100Q 2 years ago. I had Best Buy install the head unit and Infinity Reference replacement interiors. They charged me an extra $20 or so b/c of the "odd" wiring of an Audi. I have the plastic mount that sits in the center behind the head rests in the rear. It has head phone jacks and a switch. I never got it to work and have no idea what it's for. Perhaps you could find a schematic of the wiring in your Audi manual or somewhere on the web. I didn't get a manual with mine btw.
_____________________________
"I said come down here and see what happens if you try to hit me."
"Idiot!"
"Let's see what your best move is." ______________________________ "I'm a free man and haven't gotten a conjugal visit in 6 months."