I have a 72 dart swinger with factory air. It has been stuck on the a/c vent and will not go to heat or defrost. So I changed the switch and still the same. Here is what I have discovered. There is a vacuum control down low near the fresh air vent that opens the door for the heater and closes the a/c. I unbolted it from the bracket and I can move it manually to open the heater door,then the defrost door will also operate as it should. The heater door is not stuck as it moves very easy with this unbolted. I did notice there are 2 vacuum hoses to the vacuum diaphram, One has constant vacuum and the other has none when I operate the control switches. It still has vacuum when I turn the unit off. This hose seems to be sucking the diaphram ,keeping the door in the closed position, I have tried both the switches that I have and it is the same with both. ANy one with any ideas would sure help. Thanks