The Taurus/Sable has multiple problems with the HVAC system. 1)The OEM antifreeze will deteriorate and clog the system over time. A radiator flush/antifreeze is highly recommended for this model. 2)The actuator motor can strip a gear and fail to move the door correctly. 3)The plastic blend door tends to break over time and has to be replaced.
There is a video posted on YouTube that goes through the diagnosis and repair for Taurus failures with an innovative repair process. “Taurus heater” will get to the video.