I just got my heater fixed too. On my 98 f-150 the blend door motor was bad, it wouldn't move at all.
One day i was just driving and the heater started blowing cold, then it was stuck on cold for rest of the day, then the next day it had switched back to heat and was stuck on heat until today. I had to fix it for the AC to work.
First i checked for a cracked ****, but it was fine, then i pulled out the control, that the **** is attached too, and i test it with a ohm meter, it worked fine too. So then i just assumed the motor was bad, cuz i couldn't hear or see it move. So i pulled the motor out and sure enough, it wouldn't move.
So i went to the ford dealer and they had a new one for me to plug in and make sure the motor was really the problem.
The new motor worked perfect, only $82 can with tax.
I was happy it wasn't the blend door broke.
Changing the motor wasn't too bad, just pull off that black plastic trim on the tranny hump, then undo the passenger side heater duct screw, and remove the 3 screws on the motor and wiggle the motor straight down , then wiggle it out, it's a pretty tight squeeze.