Door ajar light goes on and off constantly and interior lights go on and off

My 02 Mustang's light was still on because of the door ajar button. Open the door look at the latch. The latch that goes up and down has a door ajar button hiding near this. It can be hidden by a rubber covering on the side of it. Or you can take the door inner door cover off and try to squirt some oil into the door ajar. You can try oiling it and work it back and forth. Also on the the latches sometimes they dip them in plastic. Mine was loose and would not let the door shut properly. So that was another problem with the light being on. I cut off the pieces that were jamming the latch up.

your pin switch on the door frame ,may have dirt or rust held on with small screw remove the screw and lift out the pin switch try clean it first.

If only the door ajar light and interior light is not working it is probably the door ajar switch located on the door latch inside the door. sometimes a little WD40 sprayed towards lower side of the latch will work, frees the piston in the switch. if not the switch must be replaced. If also the outside mirror control does not work, the door lock switch also does not work, it is a bad ground to those three systems, the Electric windows have a separate ground wire and still work. You must feed a new black wire from a good ground on the body thru the door accordion tube and connect it to the larger black wire on the plug that goes to the door lock switch. The door lock, the mirror control, the door ajar light, and the interior light problem will be gone.

you have a faulty auto alarm system have to have it tested

this problem is caused by a tiny micro switch that is attached to the door latch inside the doors, the stick and give this false warning, have replaced dozens of them, it is called the "door ajar" switch oddly enough :)

You need to replace the door jamb switch. Little button on the door jamb that is pressed in when doors are shut.

there is a timer on the interior lights to turn off after 20 minutes.
one of your door switches are bad,and it tells the computer that a door is open and the lights are turned off,that makes sense.
a way to test this is to undo the battery for a minute. then see if the interior lights come on. one of the door switches is bad

THE SWITCH MIGHT BE STUCK! OPEN YOUR DOOR, LOOK FOR A BLACK ROUND SHAPE BUTTON AND GRASP THE ROUND EDGES WITH YOUR FINGER AND PULL OUT AND SEE IF YOUR DOOR AJAR LIGHT COMES ON, ALSO CHECK THE FUSES AND LAST CHECK YOUR INTERIOR LIGHT BULB. GOOD LUCK!

This is usually caused by the switches sticking in the door latches. Try spraying some WD40 in the latches and open and close the doors a bunch of times that should take care of it.

The switch that tells the computer that the door is closed is broken or has a loose wire.This causes the computer to think the door is open thus the door ajar and interior light on.Switch should be located on door jam.