I was only getting heater from my defroster so I took the heater/AC control panel off and there is 2 black vaccum lines that are just hanging back there. they have female connectors on them and are connected togehter. I believe they come from the engine but where do they plug into.

 

When I swapped engines I removed the A/C box and had the same problem, I could'nt get it to switch off of defrost and go to the vents or the heater by the feet, there was a tiny black hard plastic line in the engine bay with a little rubber end on it, I stuck it on a vacant port on my carb and now the selector works again. I don't think it takes too much vaccum to operate it.