1994 Ford E-Series Van vents

its not a secret, you just have to think like an engineer. They designed the system so that if it failed it would always blow defroster and clear the windshield. those duct doors are vacuum powered, and I would suspect that a vacuum line came off or has a hole in it. If you also cannot change between hot and cold on the dash then it may be a vacuum line between the engine comp. and the HVAC unit in the dash. if it's only the duct door then it could just be a line fell off between the HVAC unit and the door actuator. Also, if the vehicle has a rear AC unit, change the knob from "vent" to "defrost" and see if the rear unit swaps from the overhead vents to the floor vents. if those do not swap and they just stay on the floor vents then your vacuum leak may be in the rear unit behind the drivers side interior panel. hope that helps.

Reserve tank or the check valve in the line from the tank into the car may have a leak...