Just wondering, does it also get wet with the a/c on?
If so it may be the airbox drain is blocked. I tried finding the a/c drain hole from under the car but didn't see it. I then found a rubber grommet in the side of the airbox on the passenger side footwell, pulled that and found gunge had blocked the hole. No more water on carpet now. But saying that (about the grommet) i think I read somewhere on this board that that may not be factory fitted and a dealer may have done the mod.
Good luck and hope you find it soon, wet carpet stinks BAD and in this weather takes ages to dry out