If it's not the light switch, you could have a bad door ajar switch. It happen to my 99GT. I don't know what goes wrong, but the car doesn't think the driver door is ever closed. Try something. Close the door and get out of the car. Use your clicker to lock the doors, and then hit lock again. If the car beeps once, then it's fine. If it beeps twice, and all the doors are securly shut, then it's a bad door ajar switch. When you hit the button twice on mustangs, as well as most fords, and the car beeps twice, it's telling you a door is still open.