[Daves55Sedan]

My '55 Ford sedan came with the dealer installed recirculating air heater which was offered to dealerships in the southern states for customers who wanted heat and windshield defrost. That exact same heater was factory installed in trucks going back some years and also offered to dealers for truck installation in southern States. They were very common in cars in southern states for a number of years but even more so in trucks.
If you can find a boneyard that still has a lot of antiques, look for '49 thru '56 Ford trucks first. If you are in the south, look in cars of that era too.
If nothing else, they can be found on e-pay periodically.
The recirculating heater was very simple and self-contained. It consisted of a plenum/housing with elbows for defrost hoses, a core and blower motor. The housing was mounted on the inside of the blank plate over the rectangular hole in the firewall that the factory installed on southern cars. It required punching two holes for the heater core inlet and outlet tubing. There was an under-dash control cable to allow hot water into the core and there was a rheostat switch under the dash to operate the heater blower motor. For the water control, simply use either of the systems (the vacuum type or mechanical type) that came with the donor car.