The ACC air mix valve is controlled by a wire "rod" that's between 1/32" and 1/16" in diameter.  It coils around a plastic rod that's connected to the air mix flap.  The flap rod can break or the control actuator can jam.  Sorry, but I don't know where the actuator is located (presumably, it's somewhere under the dash).  You'll need to remove the "aquarium" cover to see it.  The rod is pretty much dead center left to right.  It should move forward when the temp select is run to full cold with the AC on, and all the way back when the highest temp setting is selected.  The full travel is a couple of inches.  Note that my previously damaged heater box has been repaired with duct tape (I'm not THAT proud!).