Many times, when the car's heater is disabled, the light will stay on. To troubleshoot this, first confirm that the fuse isn't blown. Sometimes, the fuse may JUST need to be replaced, while other times there may be a more serious issue with the wiring or components under the hood. If the wiring has been messed up, it will usually necessitate a complete electrical restoration in order to fix it. In many cases, though, simply turning off the car's main power (by pressing the switch beside your window) and then back on again may reset the system and enable you to use your heater.