If you don't see anything obvious then it's amost always the glove box light. The door always warps because the switch spring is too stiff and eventually stops turning off the light when it is closed. If this is happeneing and you only make short trips it's not enough to keep the battery charged up especially in cold weather. You can usually solve the problem by taping or glueing a penny or two over the spot on the door that contacts the switch button.