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The dome light should illuminate when either door is open. OR You can "override" the dome light door switches and make it come on manually by fully rotating the headlight switch knob that is used to adjust the brightness of the guage lamps. Turn it until the guage lights are brightest and then it should "click". This completes the circuit to make the dome light come on even if all the doors are closed.
Sounds like the passenger side door jamb switch (look carefully with the door open, you should see a little plunger sticking out) it pops out and makes the light come on by grounding the dome light. Carefully remove the plunger type switch by using a miniature wrench--it unscrews. There should be a wire connected. Touch the wire to a good ground, light should come on. If no wire at all, or light doesn't come on, other problems
and you've got to chase some things down.