Likely a vacuum problem in the control system inside the dash. normally the 'default' configuration is to the defrost-vents when there is a problem. there is a vac-motor that controls the air direction within the plenum. that motor either sends the air flow to the dash registers or lets it pass downstream to the floor/defrost motor control vents. the panel door vac motor has a blue hose to it from the function control selector. the downstream 2-circuit vac motor has a red and a yellow vac hose to control each vac circuit in that motor. there is also a white vac hose to the outside air control motor. first thing to check is the vac check valve on the firewall in the engine compartment to make sure vacuum is reaching the function control panel.
Check for vac from the check valve to the vacuum manifold behind the function control. There is always vac to that control. The blue hose from the function controller should not have vac when the selector is in defrost/floor. If there is no vac at the blue hose then I'd say the panel door is jammed. Find the vac motor in the blue hose circuit and the panel door has a vac line attached.
Here are some excellent pictures of the heater, click on the thread. http://www.ford-trucks.com/forums/10...cs-needed.html