If you have your system on heat and you are only getting hot air from the front vents and not the rear you might have a bad actuator motor. There are two different plenum doors on your vehicle. One opens and closes for heat to the front passenger area. The other one opens and closes for the rear passenger area. There's another answer out there for this rear A/C problem advising owners that the rear control knob on the front dash is bad and replacement is 'a snap'... Can anyone comment on these two trouble-shooting tips?
The blower on the back heater may need to be replaced on the 2000 Ford Winstar.
When you have the heater on, does it just blow any air at all? or no air?
For a Windstar, a common problem is a failed temperature blend door motor - actuator. See "Related Questions" below
Because your thermostat is probably stuck open
Leak in heater core. Replace core.
The front expansion valve is clogged.
heater core possibly plugged and leaking
The thermostat could be stuck open.
It doesn't really blow it out it just warms the cold air.
heater will not blow on a 1996 ford contour
if the heater does not blow out warm air change the setting.
Low coolant. If the coolant is low then the heater core is not able to transfer heat in to the car.