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Discussion Starter #1
I have a 68 GT Convertible. The courtesy door lights, and the 2 under dash lights that would light up the floor mat area, and cigarette lighter do not work. In addition, the instrument bezel lighting is very dim which may be a separate issue. So far i have done a conductivity test on both door switches and they are fine. I have replaced the headlight switch which apparently the lighting described above, passes through. All fuses are fine in which most have been replaced or continuity check was conducted for integrity. A few years ago the driver door courtesy light wiring that runs between the door and door jam severed due to 50 years of opening and closing door. I reconnected the wires with butt connectors and it worked temporarily. The car was recently painted in which the doors were removed painted and reinstalled with new harness for driver side. I thought this would resolve the issue to no avail. A friend indicated this lighting and lighter issue could be as a result of a grounding problem. Any thoughts, suggestions or input would be greatly appreciated. Thanks, Kevin
 

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I believe courtesy lights and the cigarette lighter are on the same common fuse. And both use the body for the ground return. Do you have a ground strap between the engine and the firewall?
 

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Discussion Starter #3
The cigarette lighter and emergency flasher is on a separate 20 amp fuse. The emergency flasher does not work either. The courtesy light fuse is also linked to the "Clock and Console Rear light". The door courtesy light ground screws are located just below the heater / temp controls and are tight. Not sure what the "ground strap" is butt will check the engine to fire wall ground. Thanks
 

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Here is a simplified schematic. I recommend measuring voltage to a body based ground on a green-yellow wire 54, as well as on a black-blue wire 53.

I would probably pull out one of the door jamb switches, pull both connectors out, and measure each of those to ground. Reply back with the results.

The ground strap is a flexible copper strap from the firewall to the passenger side of the rear of the engine.

If you don't have a volt meter this might be difficult to get to the bottom of.
 

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I had an intermittent issue with the courtesy lights not working. I checked the ground strap, cleaned up the grounds for the courtesy light assemblies and the door switches and never figured it out.
I think what finally worked was I gave the glove box light switch a little twist to get it to seat better and the courtesy lights have been working since.

I can't say if that was the issue or if it was just a coincidence, but the lights are working now.
 

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Each bulb (or pair of bulbs in cases like overhead console map lights) have individual, not common, grounds.. Fixing the ground on one or two lights will fix the corresponding lights, but not any others. The glove box light twist was an effective method to clean up its ground and get it working again. As far as fixing the larger problem that you had, the twist of the assembly may have helped if that twist traveled up and affected the green/yellow circuit 54 feed. That connection might be corroded and you got a bit lucky.

With the OP stated problem of multiple lights, cigarette lighter, and dash lights, we need to think in broader terms.

Post #2 is along these lines. If you start at the battery ground connection, and work out from there, it connects to the engine. If that connection has failed the starter motor won't turn over. I'm assuming that the starting function is fine because it was not mentioned. The next link in the ground is the aforementioned engine to firewall ground strap. If that is compromised, everything that is grounded to the body will be affected, which is a lot of items.

If the OP's problem is indeed limited to those specific items, and other things (headlamps, tail lamps, radio, etc.) work fine, then the ground strap is ok.

Once we hear back with some voltage measurements as well as feedback on the strap, we can go to the next step in the process.
 

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Discussion Starter #7
So i was able to track the problem down!!!!! Approximately 40 years ago there was a small fire under the dash which was quickly extinguished. I was able to clean up what i thought was all the fire retardant. Thankfully there was no damage other than electrical tape wrapping getting singed slightly. Fast forward to this week....With the electrical problem as described above, crossing over many components I decided to remove the mounting screws to the fuse block and inspect the back side. As soon as i removed the unit about 2 teaspoons of fire retardant dropped straight out of the back and down to the carpet. The retardant was at the bottom where the 2 corresponding fuses are positioned for the component issues. I cleaned thoroughly and remounted the block. Attached the battery terminals and tested the cigarette lighter and it worked after the second try. what a relief. I next tried the emergency flasher and all the blinker lights stayed on continuously. After turning the blinker switch and emergency blinker off/on several times everything sequenced perfectly. This had to be due to inactivity over a couple decades. Excited, but courtesy lights and interior lights still not working. I checked the fuse for continuity and as soon as contact was made all the lights came on. I assume this was also due to inactivity. I WANT TO THANK EVERYONE WHO RESPONDED TO THIS ISSUE AND EDUCATED ME ON SEVERAL TROUBLESHOOTING SUGGESTIONS.. This is the first time i have used this forum and have signed up to continue participating. All the Best and stay healthy!!

Kevin H
 

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This is a powerful thread.

Great assistance + a valuable follow-up. Nicely done everybody.
 
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