Check to make sure that the vents are switching to each location, if not, then the vent selector is not operating correctly. Anyway, the problem might be in the heater core block off door. In a good number of Fords, the heater core is always supplied water, and a door blocks it off from the air plenum.
Swing the temperature selector from hot to cold and see if you hear a door closing there as well. That's probably the problem. That will be in the dash. The vacuum line to it might be kinked, or the door is simply stuck. I'll have to go dig through my manual to see where the actuator is located so you can manually operate the door.
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Moderating everything now, let me know if I can help. To view links or images in signatures your post count must be 10 or greater. You currently have 0 posts.