If I remember correctly, the temperature knob is cable actuated. You need to take off that panel and see if it's moving the cable when you turn it.
Otherwise the cable is broken, or disconnected at the vent controller. Time to tear apart the dash....sorry man.
Scott
P.S. How do you know it's a problem with the knob? Is the heater core hooked up and not bypassed? do you get hot all the time? Is the a/c running?
Scott