Just writing this as I remain suprised that there is not an obvious sticky thread for this design fault and I continue to read posts about people having problems when it rains.
Surely every A4 B6 owner has now removed the grommets in the bottom of the plenum chamber? There are rubber grommets below battery on the left and below the brake servo on the right fill with crap and dont let water escape from the car.
If you haven't removed these it is purely a matter of time before your car fails in an expensive (water in ECU, HVAC, engine) or dangerous (brake servo failure) way.
I dont think this common fault is highlighted enough in this forum as the consequences of not sorting can cost many thousands.
How to fix the problem here
(for a passat but identical problem)
### UPDATED LINK Interior leaking | veedoubleu dot com ###
Take your car to a dealer and they will fix it for free.
Really hope this saves a few people from disaster.