It could be a problem with the vaccum switches too. I had the same problem with my truck when I bought it and found out one of the vaccum switches was broken off. Here's a picture I took when I found the broken mount.
(looking up with back of head on brake pedal)
The vaccum switch on the left controls the Heater/Vent-Defrost switch, and the one that goes where that broken mount is controls Vent/Defrost. You probably have a problem, if not the same problem, with the switch that broke off on mine.
Good Luck,
Eric