Abstract: A circuit for allowing an air conditioner to be installed in a motor vehicle using the vehicle's existing heater and vent controls. A slide potentiometer is attached to the heater control arm that normally controls the heater air temperature. The slide potentiometer has two resistance elements electrically separated from each other, but only a single wiper that moves from one to the other as the arm is moved. The air conditioner and heater have separate blowers, but both are connected to the conventional heater blower switch. A relay is connected to the heater and air conditioner blower circuits for enabling either one or the other to operate. The relay is actuated by a switching circuit that is completed only when the wiper is in contact with one of the resistance elements and not the other. Another enabling circuit is completed when the slide potentiometer wiper is in contact with the other resistance element. This latter circuit enables the air conditioner temperature regulating circuitry to function.