This question is mainly about cars but it should apply equally to trucks. Ready?
You know how almost all vehicles have a grill on the cowling that is at the bottom of the windshield around the wipers? The rather large holes in these grill works allows a lot of random debris AND WATER to just pour down in there.
Where does the water go? During a torrential downpour or even in a car wash a tremendous amount of water goes down these vents. I assume the water and debris makes it down into the duct work for the HVAC. Is that correct?
Does the water that gets into the HVAC ducts exit out the same drain hole used for the air conditioning condenser?
See, I warned you it was a stupid question.