Abstract: A forced flow of outside ventilation air may be selectively directed into the cab of a truck using a specially designed fan assembly that works in conjunction with an existing vent structure disposed within a ventilation passage extending through a wall of the cab. The existing vent structure has a control member which is manually operable from the interior of the cab to selectively permit or preclude outside air inflow to the cab interior through the ventilation passage. The fan assembly includes a housing having an open front side, and an open rear side secured to the cab wall portion over the inner side of the ventilation passage. A cover plate having a pair of air flow openings therein is secured to the front side of the housing for pivotal movement relative thereto between a closed position in which the cover plate blocks the front housing side, and an open position providing manual access to the vent structure control member through the housing interior.