Interior lights, windows and sunroof do not work

I had the exact same issue on my 1999 A6. On the instrument panel I saw a symbol that looked like a door was open on my car. For mine, it was saying that the back door was opened (which it clearly wasn't). I found that the problem was with the wiring under the seat for the rear foot well lights. I unplugged both the driver and passenger side rear foot well lights (located under there respective seats). This fixed the problem right away.

From what I understand, there is an issue with the way the wiring is routed and when the seat gets moved repeatedly over the years it wears a bare spot in the wire causing a fuse to blow. Unfortunately for me, replacing the fuse doesn't fix the problem. I went through 8 fuses in under a week trying to track down the problem.