I was looking at the ( 1996 ) Ford Windstar Owner Guide and it shows : In the Power Distribution Box ( which is " live " ) located in the engine compartment near the battery ( " M " is a 60 amp fuse for the Heated Backlight , that's what Ford calls their rear defrost )
The vacuum supply line from the engine is disconnected or broken. When the vacuum supply is cut off the system defaults to the defrost mode.
There is most likely a vacuum leak on the line from the engine to the air conditioner control.
Make sure you have vacuum coming into the cab from the engine. There may be something wrong with the switches.
Check the vacuum supply from the engine. If it is disconnected or cracked the system will default to defrost only.
Not knowing what vehicle you have, many vehicles use a vacuum supplied by the engine to operate the different positions for the air to come out. When the vacuum is absent the system defaults to the defrost mode. I would check the vacuum supply hose starting at the engine for cracks, damage and disconnection.
Cooling fan is designed to come on only when A/C, defrost, or engine temperature exceeds 235 degrees
diagram of motor windstar 3.8
Do you have a 3.8 engine for a 2001 Windstar Ford Van?
The engine may be reliable but there have been too many problems with power window switches,electric motors, power sliding doors and door ajar warning lights.
There is no carburetor on a 2001 Ford Windstar engine because the engine is fuel injected
Many HVAC systems operate with vacuum supplied from the engine. If the vacuum supply is cut off the system goes into default position which is floor and defrost. Look under the hood for a small vacuum line from the engine leading into the firewall that is damaged, cracked, pinched or disconnected.