My old Audi A6 Avant had the same\similar thing happen, and if you can only feel wet\damp carpet, then you have some way to go to match my experience, like you it sounded like the rear passenger footwell, it certainly felt damp, and I was suspecting door seals etc initially, every time it rained it got worse, then each time I went out, it was like going to the seaside, watching the waves breaking on the shoreline, and I do not mean that as a metaphor, I could only take one person (2 if they were friendly) in the rear drivers side, and I kid you not, as I braked you could clearly see waves of water sloshing up in the front footwell, just like waves hitting the beach, and the same in the rear passenger footwell when I pulled away, and by the time a mate was able to get time to do it, you could see the pond over the carpet, and that was before moving the car.
The guy teaches the army mechanics, has been in the trade at least 30 years, and he had never seen anything like it before, nor even heard of such a thing, he took it away and a week later, he had stripped that side bare, seat and carpet out etc, left heaters and dehumidifiers running 3 days straight, not even close to drying it out, in the end he was forced to drill a new drain hole through the front floorpan to let the water out, he took 10ltrs of water out by the end, and he had to jack the rear end up to manage that, it took very little imagination to picture a little 2" surfer dude riding a 30' wave lol.
He went through every mechanic he knew asking what had gone wrong (it was one of those, unless you had come across it before, you'd be stumped), and he finally found a guy who had come across this themselves, turned out the model I had turned out to be a design floor, where the bulkhead comes away from the body (something like that), and when it rains it goes straight into the car, the thing that made it so hard to figure out was due to the car needing to be in a position where it gets all day sun, as it melts the mastic or something, and my house gets the sun in an extreme way, we have gone through 6 or so front\side doors it's that bad here, if it weren't for that I may still have had it today.
The killer was in Audi's genius design, guess where they placed all the cars electrics ?
Yep, the front footwell, or at least that's where the bulk of them were placed, so they were under 2" of water for must have been months, maybe a year or so in retrospect, given how long the car was plagued with damp\misting windows in the winters, and other little niggles coming in, then the air bag light come on, and he said it would take more than the car was worth by then, so it had to go, so if your car has any similar things to this, then the sooner you check the electrics are dry the better, they may be sealed, but they won't take long before they may start to cause the corrosion that mine ended up with, but as I said, 'IF' yours has anything like in my experience, I hope it's nothing like it, but I offer it as being the sort of impossible unknown mine started out with.
*edit
Dam it.
Don't you just love bumped threads from yesteryear, sorry guy's I have enough trouble knowing what day it is, let alone year, lock this bugger down, take me out in the paddock and flaming shoot me, sorry guys.
