I think jimybob might be right. I had a similar problem recently. one of the plastic things that holds in place the outer casing of the heater cable was broken. because the little plastic thing was broken the heater knob did not move the inner cable, which is attached to a lever on the heater (which is inside the car btw). i accessed the cable without taking off the glove box though. i went directly from the front: removed the ashtray, unscrewed several screws holding the small clock panel and the other plastic things covering the heater control. it was a bit difficult seeing things and working in such limited space, but i managed to fix the problem by making a little steel holder of my own.
I'm not a mechanic, but it took me only about 3 hours to figure out how to unassemble things and then put them back together.
if you decide not to do it yourself, at least you now know where the problem might be. for a start try to decide whether the feel when turning the heater temp dial is not somewhat different from what it used to be (either too stiff, or too loose). if so, it has to be cable holder.