I just found this site searching for an answer to my problem and found toastermafia's excellent illustrated walkthrough with pics of dash panel disassembly, but this did not help me fix my problem. Scion xB 2010 (manual).
My airflow selector knob is stuck in the left half side of the dial and won't go past the top of the dial. It doesn't appear to be the dial selector itself, almost like something blocking the internal baffle. Maybe a linkage has come off inside, or maybe the nine-year-old dropped something down the defroster vent??
Any other suggestions before I tear into the dash even deeper would be gratefully received!