[dsraven]

here is a pic with the cowl removed. it shows the little trough above the vent opening.
you can see how debris would be able to enter the vent grille in front of the windshield, follow down the plenum and enter the side cowl area (above the trough) and start to fill the side cowl with leaves or needles or whatever because it wouldn't take much to plug the hole at the bottom of the side cowl, the one with the little metal flap you talk about. then when a good rain come through the side cowls fill with water up to the point where it runs out on the floor, inside the truck, through the vents doors. then it can sit under the floor mat and rust away the door pillars, cab mounts, steps and rockers, floor panels etc.