The air distribution system provides fresh air ventilation for the passenger compartment in all operating modes. After being warmed to the desired temperature, the air is discharged from the outlets appropriate to the selected operating mode. When all windows are closed, the addition of outside air builds a slight positive pressure within the passenger compartment. This pressure is relieved by the pressure relief valve on each side of the vehicle in the rear pillars (utility models) or below the glass in the rear of the cab (pick-up models) after the air has circulated through the passenger compartment.