Abstract: An open door warning system for a vehicle includes an open door detector mounted on the vehicle, which detects when the door is left open, and a light module mounted on a side of the vehicle and operatively coupled to the detector. The light module is preferably aligned with a driver side rear view mirror so that the driver can view the light module through the rear view mirror. The light module includes a light source which is driven by the detector to generate a flashing warning light when the door is left open. This warning light can be seen by the driver through the rear view mirror and thereby be warned of the open door situation. The detector includes a magnetically actuated switch controlled by a magnet attached to a top edge of the door. The switch is mounted on the vehicle to be juxtaposed to the magnet when the door is closed. The switch is coupled to a power module which includes a light driver circuit, a rechargeable battery pack, and a solar cell to keep the battery pack charged.