Abstract: A plenum cover for a vehicle air conditioner of the roof-mounted type. The cover is essentially rectangular in shape having an opening at the top with elongated side walls and a base wall spaced below the top opening and being joined to the elongated side walls by opposed end walls. The base of the cover has a continuous elongated groove located substantially parallel with and spaced between the side walls. The groove in the base extends upwardly toward the top opening in the cover. The groove in the base is adapted to straddle the partition in an armored truck or taxi cab which separates the driver compartment from the cargo or passenger compartment. A plurality of diffusers located in either the base or side walls but on oppposite sides of the grooved portion of the base for diffusing cool air into the compartments located on opposite sides of the vehicle partition.