Sounds like the HVAC case seal. It leaks around the fresh air door to the bulkhead( used to be called fire wall) on the right hand side just above the blower motor. Take a light and make sure that the cawl area is not filling up with water, this is the area that the wiper motor is sitting in. If the water drains are blocked with leaves and dirt it will fill this area up with water and then go into the fresh air duct. No water should build up in the entire area, all of it needs to drain out. If this area is ok and does not fill with water than the next thing is the bad part, the HVAC case must come out of the truck to re-seal it the correct way, thats a 8-13 hr job. Not fun. To verify were the leak is take a garden hose and direct the water directly above the blower motor and see if it leaks. Keep moveing it untill it starts to leak. Could be the windshield but not the norm. This should give you some idea of were to go.
