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I'm not sure how much they have changed the heater design over the years, but my 71 5500 was not putting out much heat last winter (my first winter with it) and I assumed that I was nto getting water to the heater core. WRONG! The core always gets water on that unit, there is an AIR door that has to open to allow air to pass into the heater ducts after it is heated. This particular truck had belonged to a museum and sat a lot of the time, mice had gotten intot eh cab and had filled the air box with stuffing from the seat cushion. There was a ball in there the size of a football, completly closing off the air duct between heater core and ductwork. Open it up and make sure that the temperature control is moving whatever "valve" it is supposed to, be it air or water.