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I have a 2001 F250SD.reg. cab, 5.4L gas engine .....no air conditioner.
Various settings of the the interior fan and heater brings outside air into the interior of the cab.
One of the air intakes for this outside air is in front of the driver, on the exterior of the the truck, just below the windshield wiper. Thru that grillwork located right there, at the base of the windshield.