if its not the heater core then it might be the weather seal around your door or under the hood. I had that happen to me in my passat...by the was a P.O.S. I had a missing seal under the hood on the firewall and every time it rained or I washed my car it would leak really bad. I had like 2 gallons of water in my car one day. Do a leak check by running a waterhose over your seals and try to locate it that way. Good luck