I had the same prob on my 2000 s4 b5. It was a crappy push fit connection for the rear wash wipe located behind fuses in the front passenger kick panel. Re-connected it and taped it up with duct tape and its been fine. Hope this solves your problem.
Also depends how far you've gone checking out how wet the carpets are. If all you've done is check them from the top, then the water collects at the back, once you lift the carpets you may find the front is wet as well, just not as much. There is an inch or so of foam under the carpet which wicks the water to the lowest point. Right PITA as you have to squeeze all the foam out to get it dry. Why Audi didn't feel the need to seal the foam so it wouldn't absorb anything god only knows. I bailed out over 5 litres when my drain holes got blocked and the carpet was only slightly wet in the rear on the carpet side.
Eventually found the cause of the water leak,it was the return valve for the rear washer jet that had snapped and was just jetting straight into the passenger foot well (front and rear)which is about 3" deep in water !!!. Seems a stupid design to have the water pipe running straight though an electrics junction box to me as well !!!
Yes its supplied from the front bottle,i made a bit of a mistake though as the valve which had snapped was actually for the central locking which i only found out in the dark when the car alarm wouldnt set and the door would'nt lock. The water pipe had come off still and is just to the front of the electical juncton box ,fortunatly just a push fit so all done apart from i have a swimming pool in the front of the car which i dont think was an optional extra on the sports models !!