You have confirmed the 2 primary culprits:
Suspect a plugged up heater core then.See "Related Questions" below for more
There's the heater core itself - getting plugged upAnd then there's the blend door itself - they've been know to get stuck and/or break.See "Related Questions" below for more
If by "heater box" you're referring to the heater core, we've got you covered with the "Related Questions" below.Be sure to review the questions regarding the blend door and blend door actuator motor - other very popular heating/cooling parts in the Windstar
For a Windstar, a common problem is a failed temperature blend door motor - actuator. See "Related Questions" below
How do I locate the heater blend door actuator in a Ford f150 pickup
The most common problem with heat and a/c in the Ford Windstar is Blend Door failure. The Blend Door actuator on the Ford Windstar can be found by removing the glovebox. A broken blend door actuator is almost as common as a broken blend door itself. Typically, the fix requires disassembling the entire dash and replacing the whole plenum box, which can be a costly repair. Linked here is a video from www.heatertreater.net demonstrating how to access components of the plenum box for repair without disassembling the entire dash.
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
The Blend Door actuator on the Ford Windstar can be found by removing the glovebox. A broken blend door actuator is almost as common as a broken blend door itself. Typically, the fix requires disassembling the entire dash and replacing the whole plenum box, which can be a costly repair. Linked here is a video from www.heatertreater.net demonstrating how to access components of the plenum box for repair without disassembling the entire dash. heatertreater.net/Windstar%20Video.html This details the best DIY option on how to replace a broken blend door or actuator.
There are 3 primary questions about Windstar heating & cooling problems: * Temperature blend door actuator motor replacement * Heater core replacement * Various causes of 'no heat' problems See "Related Questions" below for links
Can't say for positive sure, but I would expect the Freestar/Monterey blend door actuator motor to be much like the Windstar. Common failure, easy replacement - about $40 See "Related Questions" below for links to the Windstar related blend door discussions
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The temperature valve actuator is located on the front of the heater case