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When I replaced my interior last year I noticed that if the metal caseing for the dome lights touched the body of the car the lights came on. If memory serves me correctly, (and that is rare these days) the three position switch controls the grounded side of the curcuit. If you look at a wiring diagram, I think the power supply goes directly to the two dome lights, and the switch controls if the lights have: 1. no ground-centered (lights off), 2. ground for door switch-lower position (lights on when doors open), or 3. active ground-upper position (lights on).

If this logic is correct, I would start by checking the wiring to see if something is grounding out somewhere. I would also test the three position switch to see if it has continuity in the upper and lower position, and no continuity in the middle position. That will tell you if you have a good switch.

The map light is a totally seperate unit and does not wire into the dome lights. The power may be supplied by the same source.

Good luck, and let us know what you come up with.

Oliver
1966 1800S