The other day I was driving my 99 Expedition had the AC on Max suddenly the system changed to full heat. Tried turning the truck off and on again with no change. The next day the system started working on it's own. This has been going on for about 3 weeks now, but now I need the heat and it won't come off AC. The buttons on the unit changes between vents, when I change temp I can hear the fan speeding up and slowing down properly. Now I am lost don't know where to look next.

 

it sounds like you may have a vaccum leak it should default to blowing out your defrost, check the engine compartment vaccum lines for holes or one that is disconnected