Look under dash panel on drivers side above gas pedal is a servo motor that turns a large roundish cam plate. With the key on, see if the cam turns in proportion to the heater control. I have seen some f-150s where the foam that seals the function doors in the heater box falls off and jams the doors up. If the cam plate only turns a little, remove the servo and cam plate(8mm screws) and see if all the levers under the cam move freely. if they don't then you probably have a jammed door and the dash and heater box need to be removed to fix it. BIG JOB!
Hope this helps
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