I have a 2013 Ford Edge Limited AWD. The heater worked on the passenger side but the driver side only blew cold air. In addition to being uncomfortable, it was a little dangerous because I could not defog the driver side of the windshield. Whedge's solution worked for me:
1. Touch Power on the physical button behind the shifter.
2. Touch Defrost while holding power. Keep holding both until you hear the actuators moving.
3. Release both buttons.
3. Touch Power.