searched up the forum about this issue and found this:
You mean the front windshield defroster?
When you run the fans to the windshield, the A/C automatically kicks in to help dry up the air helping to clear up the windshield faster. The water leaking is is the water from the A/C.
Run it on heat and just to the middle vents and see if you get the same problem.
this is exactly what is happening to me. running the fan to the windshield (A/C is off) and water is leaking under the car.
but if run the fan to car interior, then no water leak.