This is an easy fix!!! Assuming this is the problem. My stupid ?&*@ing dealer messed up so much when they installed the xm radio after I bought my truck. There is a thermostat which is to the lower right of the steering wheel. There are some slits right there which go to the thermocouple/thermister. Anyway...you can look behind (w/o pulling the panel) and see if it is plugged in! My dealer didn't plug in the thermostat, the 4WD HI/LO switch and the rear cig light adapter and left some bolts missing too.....this was a long time go and it is all fixed now.

Anyway the problem you are describing with the AC only putting out cold air is identical to mine. If the switch is plugged in to the harness then maybe the thermocouple is bad or the harness isn't plugged in at the other end....give that a look. I would bet that is the problem. You should be able to plug in the harness w/o having to take down the whole panel (I was).