Same problem with 21012 F150. Took it to the dealer, they told me that they have moved from vacuum actuators to digital actuators and if they close tightly, they strip gears. At least that's what I walked away with, and I checked a friend of mines and his is the same. Now that summer is here, I cut an old sock up and stuck it in the drivers side, the vent closest to the firewall and used duct tape to cover the other duct and the duct on the passenger side. Wish I had a good answer but as it is I was freezing my tootsies off.