Abstract: The present invention entails a power switching system for a tractor-trailer that can be selectively actuated to direct power to the tractor unit from an auxiliary power source such as a refrigeration unit mounted on the trailer. When the tractor-trailer is in a parked position, power can be furnished to the tractor and to energy consuming components thereof through an auxiliary power source carried by the tractor-trailer, thereby improving the total fuel economy of the rig. In the power switching system of the present invention, there is provided a switch assembly for operatively connecting run, accessory, start/ignition circuits to the auxiliary power source. In addition, an oil and water heater circuit can be powered through the same auxiliary power source. Finally, a trickle charge may even be directed from the auxiliary power source to the battery of the tractor engine.