Kappers probably right, the door switches send a common to light it. If it comes out of the socket, it picks up the common from the aluminum mount. One of your door switches could need adjusting. just loosen the mount and pull the center out a little.
Many times when releasing my emergency brake, it knocks the courtesy light out of the bracket a little bit. This always makes the lights stay on even with the doors shut. It may be a ground issue or something.
I've been meaning to secure it in the bracket better. Sounds like you have the same problem.
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