Take it apart to determine the cause. Only then you will be able to fix it.
Some systems use engine vacuum to move the control doors. You could have a vacuum leak.
The blend door is vacuum controlled. Any vacuum leak in the engine system can cause the door to not function properly. Check this system first. The door could be physically sticking but a vacuum leak is a more common problem.
A good place to start is to check for leaks in the vacuum lines associated with the heater controls. Many of the controls are vacuum actuated & if there is a leak in a control or associated line it may cause a rough idle
Probably a vacuum leak
Check for vacuum leak or bad vacuum motor (under dash) Check operation of heater controls
Could be a vacuum leak "defrost" is the default position of the system Check vacuum lines and motors under the dash
vacuum leak
A blocked heater hose could cause a leak. A stopped up heater core could as well. Or the connector in the Windstar could be cracked.
with your mouth
You probably have a vacuum leak. Check under the hood for a disconnected vacuum hose.
Some of the HVAC controls are vacuum operated so, yes a vacuum leak could effect the cooling.
Windstar's weren't built until 1995, so do you have an aerostar? or a totally different vehicle?