@Gordon Nichols I am trying to figure out another heat option, that being the stock heat exchangers but with airflow supplied by 12v blower. I first pictured this as simply capping the fan housing outlets and plumbing in a two-outlet blower fan, see https://www.flickr.com/gp/farsightful/F03aF0 . But now everything I read emphasizes the vital importance of having continuous airflow through the heat exchangers even when the 12v blower is off. I interpret this to mean that my idea requires four heat-tolerant 2-in diameter one-way backdraft dampers, so that airflow doesn't flow back into the blower when it is off, nor flow back into the fan housing when the blower is on.
Or optionally I need two very-heat-tolerant all-metal blower fans, that can handle heat exchanger output when heat is on but blower is off, installed in the ducts between the heater flapper boxes and the cabin.
Do you see a third way? Thanks for your insights either way.
@KenTO Apologies for parasitizing your thread, and welcome fellow newbie.
Bill