Just repaired my A4 b7 cab after getting cold feet. basically no air to the footwell vents so took off glove box and you look at the image posted earlier:
I ran all the different vent positions and it was the highest motor with the black linkage that wasn't moving.
From what I can gather, the motor with the green linkage controls the trim on the upper vents, the motor with the red linkage controls the dual bit (ie varies the air temp between passanger and drivers side) and the black linkage controls the footwell vents.
It was very easy to strip down and take apart ( everything just clips in place or together). I think the sprocket and the gear worm had become jammed a little originally as once I took the motor out it was difficult to turn which is why it wasn't working
Popped all the gears out, cleaned it all up, re-greased it all and put back together and did this:
then tested with a 9V battery making sure it worked in both direction fully, re-installed and bingo, jobs a good-un!