Heater/AC blows nothing but very hot air

The heating / AC system (the auto version with climate control) in my 2001 Taurus SEL (3.0 L V6 200hp engine, 55,000 miles) has started blowing nothing but hot air. I was driving along with the perfect air temp coming out of the vents and then it just suddenly started blowing very hot air. The fan works fine and will regulate the *amount* of air blowing, but no matter which vent combination I use, or whether I use automatic or AC or the heater function, and no matter which AC/heat temperature I set it to, I get only hot air. The air seems to be hotter than the outside air temp, so it seems like the system is not blending hot/cold to regulate the temp in the cabin -- just blowing the hottest air it can.

This is the second time this has happened. The first time (about 3 months ago), my mechanic said he was able to reach up behind the dash and wiggle the damper door until it came free. But now the problem has returned.

Do you think the problem is, once again, the damper door, which I assume determines the mix of hot/cold air? Or could it be something else? How difficult/costly is it to fix if it's the damper door? I guess I could try to wiggle the door free myself but I have no idea whereabouts to feel around behind the dash for the damper door. And I don't want to keep paying my mechanic to reach up and free the door. All ideas and suggestions welcome.