Could check rear light seals and also rear washer pipe at top of tailgate is not broken or leaking .
Another suggestion here on the same fault might be a clue where it is entering the car
" there was a damp smell in my car for while and I found today a lot of water in the boot where the spare wheel is, probably up to the middle of it (yes lots of water). That was causing wetness everywhere else at the back and the water was already reaching the rear seats.
Eventually I found how to drain the water from the wheel and I think I found where the water comes down from when it rains. There is small vent just below the rear lights from the inside. The water seems to be accumulating there but definitely coming from somewhere above and I believe this to be from where the scuttle drain comes from the roof. "