Ford Repair: 2001 expedition blows only out of defrost vents, vech, air conditioner

Question
My 2001 expedition blows air only out of the defrost vents when I turn on the air conditioner.  How do I begin to attack this problem?

Answer
well it sounds like u might have lost vacum go to the head unit , i would check for a black and white or gray vacum going inside the vech, from the right side of the engine(passenger side) if it is pluged in outside look under passengers side to see if the hose is not plugged in