[gmtech1]

Uh,what? Not 100% what you are after, but there is no light to tell you when to replace the thermostat. Are you having a over heat problem? or is the car slow to warm up? These could be caused by a stuck open or close stat. If you are not aware of the thermostat ever being replaced, it would be a good idea to to so.

Ahhh...after reading again, you are referring to the temperature control on the HVAC control head. Yes, it should be lit up with headlamps on, if it is not, and all others work on the head, then the light for the temp control is out. The head will probably need replaced. I don't think you can just replace the bulb.