There is a blend door motor on the pass side of the vehicle which varies the hot and cold air. this motor has quit working, most likely the plastic gears inside the blend motor. it will have to be replaced with a new unit.
these are available through most parts houses.
RepairPal Certified shops can diagnose and repair your 2005 Mercury Mountaineer. They provide transparent pricing with a minimum 12-month or 12,000-mile warranty on repairs and services.