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Check to make sure the engine vacuum is getting to the heater/ac control. Bad vacuum hose or leak. Many vehicles use engine vacuum to move heater doors and controls. When the vacuum is absent, it usually defaults to air blowing out the AC vents.
Does it only change when going uphill, or under heavy acceleration? Or is it just when ever it feels like it? Generally, it's vacuum controlled and on a Ford, loss of vacuum when accelerating or pulling a grade under heavy loads can be normal, causing the air to come out of the defrost. To answer your question, the blower door is located in the dash, generally behind the radio area.
The problem is with the blower door actuator. It's vacuum driven and controls the air flow from the defrosters, vents, and floor. With the AC on, open the hood and check the lines (very skinny rubber lines) at the firewall in front of the passenger and work back ward. You're looking/listening for a vacuum leak, either a pinhole or maybe a worn end in need of repair.
Check for vacuum leaks under your hood. You probably have a loose, broken or damaged vacuum line. Be sure to check the one that goes to your vacuum pot as well. With your engine running, wiggle as many hoses around as you can see, and listen for a hissing sound.