Here is the RH side fresh air plenum - there is a similar setup over by the parking brake for the driver.
There is a flap that can be opened to allow fresh air into the cabin without the use of the blower (or passing through the heater core - blend doors tend to leak and allow some heated air to pass). It just flows into the truck as a function of road speed.
The plenums start at either end of the intake cowling that runs across the truck - under the windshield. They drop down past the flaps (the RH side also has an opening that feeds the blower) and terminate at about the level where the door bottom is.
There is, by design, an opening at the very floor of the trunk to allow any entrained rain, car wash, melted snow that comes in with the fresh air to fall to the ground.
If these get plugged up with rust, leaves, dead mice, etc - the water will rise and eventually spill out of the vent flaps. Poke them out from inside the vestibules with a coat hanger or something similar - or blow up from the bottom with compressed air.
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