It could be a few things; Check and make sure the carpet is not wet underneath. The leak may be up front and going into the back as most motion is forward and most leaks happen up front. If you find this to be the case, the airbos vent in front of the windshield has let small debris get through. In time this clogs weep holes at each end of the box which drain just over the front fenderwells in the rear of them. You can try to clear them from pulling the inner fender liners out, the cowl in front of the windshield, if it is removeable, or removing the fenders. Once the weep holes are clogged water backs up and spills over into the fresh air passage just in front of the passenger footwell. So check it for wet spots or signs of leaks.
If the carpet seems dry, the fuel tank it under the rear seat and it's access panel is there. It is to allow you to remove the fuel level senser and the fuel pump in some cases. It has a seal to keep raod debris and water out off the car. It could be loose or leaking.
The other issue could be rust in the fender wells or near that area.
The best thing to do is to remove the rear seat. There is usually one bolt that holds the lower rear seat in, have to remove upper parts first sometimes. Once this is removed you lift up the rear of the seat and there are curved hooks that keep the front down which you just line up until you can pull it straight up and out. The leak source may be more apparent then.