I've lurked on here for a bit, and found it an excellent source of info, so here's my first attempt at putting something back.
I'm the new owner of a 2000 S6, which had the usual door-microswitch issue ('they all do that, sir...') and decided to have a look at it this evening.
I found a couple of threads on other BBS explaining how to remove the lock and split it on the bench, and set about it.
After much faffing around, I decided I really didn't want to remove half the door-guts unless I absolutely had to.
However, assuming that the problem is JUST with the microswitch - IME, you don't have to remove much at all
This is how I did it on a 2000 S6 avant, so YMMV.
Remove the door card (-2x pozi screws in the upper corners) and then lift it upwards.
Unhook the internal door-handle cable.
If you're lucky, you don't even have to disconnect the wiring for the window-switches; I could turn my door card 90deg anticlockwise and it rested on the ground with no strain on the cables at all.
Undo and remove the two M8 12-point spline-bolts that hold the lock in place. Don't use a Torx bit.
If you look at the bottom of the lock inside the door, you can see a 3/4"-wide spring-clip, with red and blue wires going into it. That's the microswitch.
You can unclip the spring-clip with your fingers, if you've got small hands, and then wiggle it out towards the bottom-left corner of the door.
You can then wiggle the microswitch out (noting the two bumps on the case that line it up in the lock) and then poke it out through the 1"-square hole nearby
With a very small screwdriver, you can then carefully unclip the 4x little clips that hold the microswitch body together, and then clean the inevitable gunge out of it. I just used WD40, as I didn't have any contact-cleaner to hand. Nothing tried to ping itself to bits, but be careful anyway.
You can test it by gently pushing down on the coil-spring inside. If it's working, you'll see the puddle-light in the door-card going on and off.
If it doesn't work, try cleaning the contacts with a tiny bit of wet'n'dry. Mine was fine after I just cleaned all the gunge out.
Then, just clip it back together. The t!t on the microswitch 'cover' goes over the spring.
Make sure that the 2x bumps are seated in the lock, and then refit the spring-clip, pointing it towards the top-right of the door, and just finesse it into place until it clicks.
Don't be tempted to refit the spline-bolts before you've refitted the spring-clip; you need the extra wiggle-room to refit the clip. DAMHIK.
Line the lock up, refit the spline-bolts, and then reassembly is the reverse of the above, in the immortal words of Sir Haynes of Manual.
Hope this is of some use.
Al.
Thanks for that. I'll add it to my useful procedures topic. 
