How to turn your heater core off and on in the cab.

[Lonestar10]

They do make electric valves for such occasions don't know name but it's similar to what you did only when factory installed as soon as you select AC it shuts then reopens if defrost or normal heat/vent is selected. My truck has a water spicket type valve but I can't use it due to apu so what you posted sounds simple enough to try.