I had this problem with my Brava. The seatbelt was soaking and there was no sign of water anywhere else. I couldn't find a leak or anything. I traced it to the sunroof drainage holes which were blocked. The water was sloshing about and trickling down the back of the seat belt. Unblock the drainage holes. Obviously if you don't have a sunroof then it's not that and I'm talking crap again...