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The cowl is probably clogged up with pine needles, acorns, dirt etc. Carefully--and I mean carefully, remove the phillips head screws securing the cowl vent panel to the cowl. Use a new #2 phillips tip or you will be sorry.) Any screws that won't easily break loose be sure to tap with a hammer. If you strip any of these screws ... well, I don't even want to go there.
Now, the drivers side you can look down where the cowl meets the fender and see all of the debris that has fallen through the cowl vent panel and ended up near the back of the fender. This can all be vacuumed out.
The passenger side is another matter. The best way to clean out the passenger side cowl is (are you sitting down?) is to remove the HVAC unit from the cowl. That's the only way to really get to where all the debris are located.
It could be that the cases are cracked (I doubt it.) The gasket sealing the cases to the cowl could be leaking as well. These HVAC units are not very sophisticated if you know what I mean.
Plan B on the passenger side: remove the fender. There's a small rectangular opening at the bottom of the cowl. You can fish out what you can and then pour water into the passenger side of the cowl to flush.
Rezvani's Latest Post-Apocalytic Monster Is a Ford F-150 Raptor Underneath
Slideshow: Called the Fortress, the 850-horsepower pickup combines Raptor underpinnings with military-inspired features, survival equipment, and a starting price of $285,000.