I posted this in the A6 area but maybe you might know something

There is a pool of water in the spare tire well on my 1998 A6 - no apparant leaks in the floor or surrounding area - any idea where it might be coming from. Like all Audi's it has the unique feature of having any water on the trunk drip into the trunk upon opening, but this is more then could be coming from there

 

From my experience I have seen the seal that goes around the taillights harden or crack and let water in. Also the tail light assembly itself could also cause water seepage. Plugged water drains for the sunroof could also be suspect if equipped with one.

 

The trunks own drain plugs could also have blockages. These are usually behind the rear wheels, and appear as small black tubes.