The first thing, as mentioned is to look for something a previous owner hooked to it. Like aftermarket radios etc. Also remember your interior lights run off that circuit too. If you don't have an ohm meter to chase it down, start unplugging stuff. Start with the dash pod, since you will need to access the wiring anyway. Unplug it and then turn on the head lights. Make sure you tape the button down on the door switch when you do to eliminate that part of it.