Why would the heater blow cold air in the front compartment but gets heat in the rear when turning on the heater what is the problem and how do you fix it on a Ford Windstar?

Answer 1

By your description, you have confirmed the thermostat is working.

That leaves a couple of other suspects to investigate:

• The most common issue with Windstar heating/cooling: temperature blend door actuator motor. The majority of time there is a clicking or thumping/bumping type noise from the center dash area. That's the motor attempting to move the temp blend door. This $45 part is a 30 minute or less replacement is the way to go.
  
  
• Another contributor is the heater core. "Assuming" no leaks or smell of anti-freeze in the interior/cabin, a core can become plugged up.
  A quick test is to find one of each heater hose - 1 line going in, and 1 return line - they should be about equal temperatures to the touch (the coolant flows through the core all the time).
  These can be flushed, as should be the entire cooling system anyways - and of course a worst-case scenario is having to replace it. Not such a horrible undertaking as in a Taurus/Sable for example.
  
  
• Close related to the actuator motor is the temperature blend door itself binding-up or breaking. Not unheard, but not nearly so common as the actuator motors.

All of these, and solutions/fixes are covered in much greater detail in the "Related Questions" below.