I'd hasten to add that I am not an electrical expert, but I can relate your experience to what happened to me a few years ago. In those days I drove TVRs, and (despite their general reputation for unreliability) I got on well with over a dozen of them. But my 1999 S3 suffered from a number of poor earths, most dramatically when one day everything failed (apart from the radio which was on a bypass circuit from the battery, so kept working). Fortunately I was close to home on a B road, and the chap from the AA replaced the main earth cable to the chassis with a nice fat new one. Thereafter all was fine, so if you haven't tried that it could be a quick and easy fix.
