Years ago, I had a 68 Plymouth which had an external propane engine heater. As I recall it was built by Phillips Industries in Minneapolis.
There was a small propane tank in the trunk, a heating tank inserted into the lower radiator hose, which was fired by a glow plug. No extension cords needed, it used the 12 volts from the battery to fire off using a glow plug, then would run with no electric usage. The on off controls were also under the hood.