Airflow Dial Stuck

What is your question?

Bought this car used at 134k. When i bought it the A/C & Heat worked fine, but one day it just locked up. It only turns anout 1/4 of the way around and will only blow from dash and dash/feet. This means I'm stuck with no defroster, and have had serious problems with fogging if i use the heater. I have a repair manual and know how to remove the climate control unit, but i have no idea what to look for/how to fix once I do that. Any advice will be appreciated.