heater

check for a vacuum leak in the engine compartment.

Fordtech is right, defaulting to defrost mode is classic vacuum issue with Fords,, there's a vacuum leak somewhere, also once I ran across where the linkage fell off the blend door, resulting in same..vacuum leak could be at the function control valves, they are combo i believe of electric and vacuum... I had an older Taurus where if you were climbing a long hill at highway speed, it would lose the vacuum supply to the function control and it would revert to the defrost mode, heat a or a\c,, I even tried putting an extra-large vacuum storage tank in the line still no good, the low engine vacuum climbing the hill would take it all away