Yeah, it is a common problem with most F150's, Expies from way back. In the 06 and older, you had to take the dash apart to get to the door/motor. The blend door motor is a $2 toy motor with a 25 cent plastic gear mounted on it. These gears break and cause the problem. It's a bad design. Some info that may be interesting:

https://www.ford-trucks.com/forums/8...-explorer.html

Free Ford Blend Door Fix