Well, there's an easy way to tell if the stepper motor is doing it's job, or perhaps there's something blocking the flap itself. Simply
- Remove the passenger side partition cover - 13 mm socket for plastic sockets - quarter turn only!! (just a simple lever that 'locks' the cover in place by swinging 45 degrees, doesn't screw on
- Disconnect the fresh air sensor - connector has a little lever to push in
- Remove the fresh air filter cover - held on the same way as the partition cover above)
- Switch the A/C from outside air to inside air and back, you should see the flap/door open and close
You may find a blockage or evidence of water intrusion (bad). If the door doesn't move like in the attached video, there's probably something wrong with that stepper motor. If it does move...
You can see mine opening in the attached video. The white part behind the door is the blower motor fan spinning.