I had/have the same problem, mine turned out to be the Blend door module (not sure if I'm calling it by the right name). It's a small electronic box that rotates the Hot/Cold door to control temperature. It's located under the Heater box just to the right of the Trans Tunnel. You can unplug it, and check the voltage at the harness. The voltage is variable in relationship to the position of the temperature knob on the heater control panel. I can't remember the voltage #'s, but each notch on the knob changes the voltage, this in turn tells the module to rotate the door. I thought my Blend door was stuck, and this broke the plastic coupling between the module and the door but it turns out the module itself is bad and just wants to keep rotating. I don't know the part #'s or if it is even listed in the parts drawing. You may just need the plastic coupling or you may need the module like I do. If you have to remove the Module, it's a PITA. There are three screws to take out, two are easy, the last one is jammed in the back, you need to use an L shaped Phillips. Good Luck! If you remove the Module, you should be able to change position on the door when the AC/Heat is off, it's harder to turn it while there is air flow pushing on the door. This will give you either full heat or cold only, nothing in between. I've been running mine like this for a year.
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