On my dome light (97) the switch side is the 12v side and I read full battery voltage. I used the right hand Phillips screw as my ground.
My LED light is polarity sensitive. It only lights installed one way. If you rotate it 180 degrees it will not turn on.
The right hand side of the bulb socket is the ground side. Reading ohms on your meter you should see low resistance to the screw head just below it with any door open. Kilo ohms or less. I read anywhere from 40k to 9k with the door open. Pushing in the switch makes it go infinity.
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