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1966 Lemans Convertible. Rear courtesy lights in the armrests and the console light only dim when the doors are closed (or when the door switches are manually pushed in) but do not go out. Had to remove the bulbs to prevent battery drain. I’ve removed the door switches, cleaned, removed some paint in door jamb for good ground, even removed + wire in back of alternator to no avail. Grounding issue somewhere, any ideas? Thanks in advance.
 

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Yep --- although those door switches appear to be identical, they aren't. If they've been removed and reinstalled at some point in the car's past and weren't installed on their original "sides" (passenger side vs. driver side) it can cause the behavior you're describing. Power comes from the source and to the "first" switch in the circuit (I forget which side is first). That switch has two positions and has two wires connected to it. Power in, and power out. When that door is open, it connects power to ground and turns the light on. When that door is closed, it sends power through the "out" terminal to the other switch's "in" terminal so that when that door is open, it turns on the light. The way the circuit works is that the 'first' door either turns on the light or sends power to the other switch. The 'other' door switch is only 'live' if the first door is closed. If memory serves, you can also get this behavior if the wires on the 'first' switch are connected backwards. On my '69 they're both white wires which can lead you to think that they'll work connected to either switch terminal --- they won't.

Bear
 

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Great explanation Bear, thank you. I did unscrew the switches and pull them out about 4” or so, and both appeared to have only 1 white wire connected to them. I did not see 2 wires on either one. Maybe there are 2 wires going into 1 behind a kick panel? I’ll check for that next, thanks again.
 

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Great explanation Bear, thank you. I did unscrew the switches and pull them out about 4” or so, and both appeared to have only 1 white wire connected to them. I did not see 2 wires on either one. Maybe there are 2 wires going into 1 behind a kick panel? I’ll check for that next, thanks again.
66 only had a single wire to the door switch, when the door is opened the door switch shorts to ground and that white wire supplies the ground to the dome lights, the dome lights receive a constant un-switched +12 volts, the second source for a ground is thru the light switch, when you turn the light switch counter clockwise the lights will also illuminate. It is possible the light switch may be bad or a white wire somewhere under the dash is shorted to ground which will cause the lights to illuminate. Try unplugging the light switch and see if the lights turn off.

Go to this thread and look for the wiring diagrams for the 66;

https://www.gtoforum.com/f50/gto-wiring-diagram-scans-22042/
 

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Thanks Randy. I was able to confirm 1 single white wire to each door switch as you stated. Tried to follow them as best I could by looking under dash, both wires appear intact with no cuts, etc. I’d like to unplug the headlight switch as you suggest, is this possible without removing the dash? If not, I may wait until the winter.
 
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