Open the glove box, push in on the sides where the "stops" catch behind the face of the dash and allow the glovebox to swing down toward the floorboard (your sh*t will fall out so move it).
Look directly behind where the glovebox used to be. If the newer trucks are still like the older ones, then you should see a white plastic square looking guy sitting on top of the HVAC duct with a wiring harness plugged into it.
Key the truck on and switch your heat setting all the way cold and listen for that motor to run. That motor is responsible for moving the blend door back and forth. If you don't hear it running when you shift the heat setting from full cold to full hot and vice versa, then the motor is burned out, come unplugged, so on and so forth. If you hear it running but it sounds like broken crap, then the gears are probably stripped or the blend door is broken in some way. If it's the gears you can replace the little module. If it's the blend door, you'd have to see if you could access it, if not it really only takes about 10 minutes to completely remove the entire dash as a unit.
If all sounds good though, and you are pretty sure the blend door is in fact moving back and forth, then next step is removing the lines from the heater core out under the hood and blowing back through it with a water hose to get the little black "crud" out of it, as it's probably stopped up.
Again..... all the above is assuming the newer trucks work in the same manner as my 99 in terms of the HVAC location and function.
