Passenger gets maximum heat all the time

My 97 Lesabre Limited has the dual heat and air conditioning. The driver's side contols work properly but the passenger side has maximum heat all the time. I removed the glove box and found that the actuator for passenger heat control is not working. What are the possible causes of the problem?

They just have plastic gears that wear out over time. For those who wonder what the actuator looks like, see below.

http://files.automotiveforums.com/gallery/watermark.php?file=/503/297374AC_actuator2.jpg