1997 Ford F-250

I am wondering if the cable connected to knob on dash is responsible for changing cabin temperature from cold to hot. I was working in Miami and am now in N. Ga. In Miami I was using the air conditioner, now that I am home I need The heater. Everything has always worked fine until now. I removed the glove box so I can see the heater box. The cable ends at the top of the box and appears to go into a little white "arm" on top of the box and recessed a little bit. When I turn the knob is this "little white arm" supposed to move back and forth? If so can I move it manually with a pair of pliers without damaging anything?