I have a leak on my 1998 A6 2.8, and you get to the reservoir by removing the wheel and taking off the wheel well liner (really not too bad a job). My problem seems to be at the washer at the bottom of the tank and I'm in the process of replacing it. Here's a link to the site where I found the seal.
http://www.autohausaz.com/catalog/lookup_p...FToken=95981375
Your problem may be similar. Hope it works!