a trick I've used for finding the tougher leaks is to sprinkle some powder (baby, flour, etc) around in the suspect area. the water will make tracks in it and lead back towards the source. any al all openings are considered suspect and need to be inspected. windshield, rear window, heatercore, any hole in the firewall, doors, etc. I once had a windshield leak that was in an upper corner and trickled down inside of the a-piller and then dumped onto the floor at the bottom by the ebrake. if I still had a-pillar trim in that truck I might never have spotted it.