If its a wagon we are talking about, I have had a few where the rear doors have come out of adjustment, or changed shape from being slammed, or endless rough roads. The immediate cure is to put a piece of adhesive rubber on the point of the body where the switch contacts. Sometimes packing a washer under the switch is possible.
To isolate it, unscrew and disconnect the door switches one by one, ensuring that all the other doors are closed while you have one open to do the switch. As soon as the light goes out when you unscrew and disconnect the switch, you've found it. All this assumes that all the interior lights are set to "Door" position.
Its worth sometimes removing the lens from both lamps (wagon) and checking that the contacts arent loose or sloppy enough to contact earth and stay on. Also ensure that the earth screw is in the right hole, and that the globe ends havent jumped out of their contacts and gone to earth. If the problem wont go away, unscrew the lamps and let them hang, then check the door indicator on the dash, if this is still lit, then look further into wiring, if not, refit the lamps and check the indicator after each lamp to narrow down which lamp or area the fault lies in.
If that doesnt work, its likely to be an interior light timer or possibly a central locking or immobiliser/alarm fault.
