Why would cold air be blowing out when heat is supposed to be on in a 2002 Mercury Sabel?

The Sable and Taurus uses a motorized unit that controls the blending doors for heat and cooling. You have to open/remove the glove box door to gain access to it. It is square looking and normally stocked at your local dealer. If memory serves me right this unit retails under 50 dollars. Has 1 plug-in and I believe 2-3 screws hold it in place.