I had an Audi A4 from 2000, and one day I experienced the most intriguing thing. I found lots of water under the front-right passenger seat. This video will help you fix this problem.
If your Audi A6 1999 is experiencing water leak, you can fix it without going to the dealer. This is a usual problem with Audi, and this tutorial will work on A4, A4 B5.