1999 Ford Windstar

I am by no means a mechanic, but I had this very same problem shortly after buying my 2000 Ford Winstar. One morning I used the defoggers to clear the windows, and when it was done, I was going to set it to a low, cool AC fan for my drive, only it was 'stuck' at max heat. I took it to the dealership later that day to find out there was a bad wire that connected the vent system to the computer. When the wire went bad, the computer stopped acknowledging the changes I was making to the temperature controls. Wire had to be replaced. Fortunately, this was just a week after I got the vehicle.