Just in case it happens to you.
When I got my truck the vent selection switch was hard to turn but worked. Then later it was like it was welded, it would not move at all. I took it apart and put it back together and it worked fine, then it was frozen again. I said "now this is weird" so I too the whole thing out (totally easy, it just snaps out) and looked at it real good. Looked fine. While I had it out I put the selecter **** back in the slot on the dash without the control and it would not turn. Ah hah! I looked at it with a magnifying glass and saw a coating on the **** and dash. I cleaned both with alcohol and then sprayed some PB blaster on it. Works great now. What was it? My guess is the previous owner spilled a beer on the dash and the beer turned into a type of super glue that only worked when it got hot enough.
I hope it was beer, if you know what I mean.