most fords do this...the heater cable (like a bike brake cable) becomes dislodged or kinked where they are attached to the control knob or down at the flap where it warms yer feet - lie down with a torch and look for the vent that the air flows out of .try that 1st while turning the knob. if its ok there...need to pull the heater control panel off to check am afraid (If knob is easy to move its likely to have come loose...if hard to turn the cable is kinked-straighten with pliers)
David, March 2011