What is wrong with my 98 ford windstar ac only blows from defrost on vents?

Sounds like the motor in the blend door is not working. There is a box under the dash that directs the air to the selected ducts. A door moves in the box to direct the air and is powered by a small electric servo. Not sure if this vehicle is old enough but some are also operated by vacuum. Either way you are looking at having to get up under the dash to find the final culprit.