When you purchase the base Re-circular heater in the 1940’s and 1950’s most did not come with the defroster outlet. This standard heater usually forced air to the floor (not on the windshield) to keep your feet warm and gradually warm the cab interior. It worked fairly well and to the driver it was so much better that a generation before when heaters were almost nonexistent.
The problem with these heaters: They warm the humid interior air from passenger breathing and did not use the dry outside air. On a very cold day with one to three people in the cab, the windows soon fogged from the breath of the passengers. The fog might even freeze on the glass.
Standard equipment for many was a rag on the seat to keep portions of the windshield clear for driving. Opening the window to get in dry air was not a consideration. It was cold outside!
General Motors and a few auto part suppliers soon realized there would be a demand for an accessory steering column mounted electric fan that blew more warm air on the driver’s part of the windshield. A good idea when all was operating correctly, however probably never worked well in very cold climates.
The attached photos shows a General Motors fan with rubber blades. This material was to either protect the blades or keep driver from cutting fingers on metal blades.
NOTE: There was even an aftermarket fan that did not use power from the 6 volt battery. It operated from the air vacuum created from a hose to the intake manifold. What a unique idea especially for the car or truck owner having little power left in his older battery!
Yes, we have come a long way in heater design!
Re-circulator Heater (No defroster outlet)
Steering Column Mounted Fan