This is my 1st car that has a plastic flap over the heater core, rather than a water flow valve, hooked to a thermostatic switch of some sort, or even to just the heater switch. I gather that the setup in my '83 is such, because the heat still tends to come through, especially at road speed.
Can I rig up a valve, that interrupts the flow through the core, until there is power to the blower fan, or alternatively, vacuum to a heater door servo? Will the coolant flow properly through the engine, with the heater core flow blocked off? Or, can all this be avoided by tinkering with the length of the temperature switch cable?
P.S., the duct flaps seem to be doing their thing, because the air goes where the selector switch tells it to go.

Once again, thanks.