You probably have a faulty temp blend door motor assembly on the front unit. It is located directly behind the radio, way back in on the front of the heater case.
Could also be the door inside behind that assembly is broken. The door lifts to allow hot air and drops to close to send cold air. I am in the process of fixing mine and it aint pretty, but cheaper to get the assembly than to replace the door which requires the whole assembly to be replaced. Lots of luck with it.
This "flap" is the temperature blend door which directs air over & through the heater core.The "Related Questions" below are intended to help explain some of the basics of Windstar heating/cooling functions and terminology.That should help in the investigation of the specific problem on the vehicle, and ultimately leading to a fix.Start with the "Where is the heater control valve located on a Ford Windstar" question and then move on to the "What could cause a Windstar to have no heat - or not be able to adjust the temperature"
Diod you check front brake pads
Heater hose or heater core.
Not a whole of symptoms to work with here...The most common cause of "... heater not working..." is a failed temperature blend door actuator motor.See "Related Questions" below for more
You probably have a faulty temp blend door actuator in the front. It is located directly behind the radio on the front of the AC/heater case.
If it's the front passenger side ,I would say your heater core is leaking .
The most likely is the vacuum line used to operate the swinging doors to redirect airflow is broken or not in place.
I would first check either the controls to the front heater core or a blocked front heater core. It could also be a blocked or missing heater hose to the front heater core, although that seems quite unlikley.
Because there is no heat in either front or back, I would suspect the engine thermostat first. If it never opens, the coolant is essentially prevented from getting to the heater cores. See "Related Questions" below for more
Having it on "PARKING"
The front expansion valve is clogged.
Most likely it's the heater control valve. If the heater control valve leaks a little it can allow a small amount of heated engine coolant to circulate through the rear heater core.
is this a windstar minivan? in colder climates many cars have a block heater to make them easier to start in winter , if it is a male electrical plug in or near the engine compartment that's probably what it's for
if the rear ac is working then you probably have a damper door that is inoperative. this is in the heater/evaporator box. so if it isn't working then you will be heating and cooling at the same time out of the heater box.
Since the actuator is mentioned, then I would turn attention to the temperature blend door itself.These are known to bind up or even break.Then there's the possibility the heater core is plugged up.See "Related Questions" below for more
It rusted out
A serious vacuum leak could cause that.
a bad water pump could cause low heat. The heater uses the coolant from the pump to generate heat.
low water, bad thermostat, vacuum door not working, clogged up heater core, heater valve broken........
The year, make and model info would help but generally the heater blower motor resister is located close by the heater blower motor.
windshield leaking heater core or heater core hoses.
Start with checking the fuseMove on to checking the wiring going into the motor.And finally, you may end up with a new wiper motor.See "Related Questions" below for more