I had this problem until someone told me how to reset the system. It worked great and I didn't have to pull the underside of the dash apart, here is what worked for me;

Turn the ignition switch to the OFF position.
Remove the RDO BATT fuse for 10 seconds.
Reinstall the RDO BATT fuse.
Turn the ignition switch to the ON position.
Ensure that the air mix door actuator cycles through the hot and cold range.

Full air through the defrosters now.