It could also be the plastic trim panel that seal's off the fresh air intake duct, they are prone to leak, if you want to check it out then here's what to do. First remove your windshield wiper's and then your wiper cowl, then on the passenger side under the cowl panel you will see one portion of the cowl that has a piece of sheetmetal covering it this piece is held on with seam sealer, you need a heat gun and a scrapper heat up the seam sealer to remove the panel. When the panel is removed you can then see the plastic bezel that keep's water out of the cab and hvac box, the factory seal goes bad in these truck's alot and leak's water in to the cab, just silicone or rtv all the way around the plastic bezel and it will not leak for a very long time. When you done resilicone or rtv the sheetmetal panel back down and reinstall wiper cowl and wiper's.