During the early years of Chevrolet and GMC truck production, the franchised dealers sold and added extras. These are referred to as accessories. One of the most popular items in the group was the fresh air and re-circulater heaters.
Both pre-boxed heater designs included a pair of defroster ducts. Both were connected to the body of the main heater box by air transfer flexible hoses.
A mechanical switch would change air direction from floor to the defroster ducts. Their long narrow openings forced heated air to most of the windshield inside surface. Thus, the name “Defroster Duct”. They quickly removed the condensation from human breathing on a cold day, and usually would, in time, melt ice so the wiper blades could do their job on the outside of the glass.
Back to the ducts! The GM trucks always came from the assembly line with defroster connectors under the dash. The dealer’s mechanic would have no major problem permanently attaching the ducts. He could not make a mistake or they would not fit. Of course this could only happen once the mechanic was lying on his back on the truck cab floor with a shop light and screw driver in hand. It is always dark under the dash.
NOTE: The more experienced heater installer always wore goggles. He never forgot the time when attaching defroster ducts, dirt that was once dust accumulation, dropped into his eyes as he bumped into a nearby part. That greatly lengthened the time on this job!