The main ground is a thick brown wire that resides in the black 8-pin connector. This lone brown wire that you describe is known as alarm ground and is usually identified as DWA-GND. It only appears on 2000 and 2001 A2s and is used by Generation 1 head units to trigger the car alarm in the event of stereo theft. Do not use it as a method of grounding equipment. If you're fitting a later Audi head unit or an aftermarket unit, just leave it disconnected.