The door jamb switch that activates the door ajar light has become corroded or defective. Cleaning or replacing the switch should remedy the problem. The part is available at your dealer and most part stores. Autozone has the part for under $12. Autozone's part number is XXXXX and it can be ordered on line by clicking here. Replacement is easy only requiring on screw to remove the part. Do not allow the wire connected to the switch to fall back in the door jam. If you have more questions please ask them.