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The door ajar switch is located on the bottom side of the door latch.
CLICK HERE FOR DIRECTIONS
If the light does not go off with the switch unplugged, you have another switch that is bad. One simple repair that works 50% of the time, you can spray penetrating oil, WD40, into the door latch and open and close the door 10 to 20 times. This sometimes cleans the door ajar switch. Most of the time it is the switch on the drivers door that ends up failing the most.
The picture is the door latch removed from the door. The switch is located on the bottom of the latch.
Below are the directions for removing the door latch.
If you have a local mechanic you trust, then that is a good idea. Without removing the latch, it is a tough job to do.