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

I'm assuming the "switch" you replaced is what is supposed to be releasing the vacuum on the other side so the door can close? Kinda WAG ing here but can you pull that line off at the switch & see if the door then releases? I'm assuming the hoses ain't turned around, plugged in on the wrong nipples (down at the servo). As you're doing I would concentrate on that one line that should be releasing

Yes the switch I replaced is the push button vacuum switch. I also thought to switch the 2 lines but there is no way the upper line would reach the lower nipple where that line has constant vacuum.

if you pull the one line (up top) will the door then close?

My truck does the exact same thing. With both hoses connected its stuck on the AC vents, if I disconnect the rear one and plug it then it will switch to heat/defrost. When I reconnect it and switch to AC it moves back and stays there. In the winter I leave it disconnected for heat and reconnect it in the summer for AC until I can find a good used actuator.

Like Robert said pull one and plug it and see if anything changes.

No rapid it don't close, I unbolted it from the bracket for now and i can slide the door open and close. Sorry for the delay reply