Electrical Troubleshooting help please

[Fast67VelleN2O]

I am having an issue with my 1967 Mustang Fastback GT. When the ignition is off the tail lights (not brake lights), license plate lights, and dash cluster lights stay on. (This is with the light switch in the off position). I went out and bought a new light switch, thinking that it could possible have been the problem as it was the original switch. Plug it in, still the same problem. When the light switch is completely disconnected from the wiring harness, the tail lights and license plate lights are still lit, however, the dash lights are not lit anymore. Car is all stock, all nice original Ford wiring. Any suggestions?

[mockingbird812]

Is all of your "nice original wiring harness" and wire plugs in good shape? Do your wires from the back of your car terminate in a plug which plugs into the light switch? Are the lights at the back of the car grounded well? It sounds like your tail, and license lights wire are connected to an always-hot power lead at the light switch. Have you double checked your connections at your light switch? Not sure why the dash lights were always on before and not now. Do you have a wiring diagram for your 'stang? Very helpful.

[elonblock]

Was this something which just happened? Have you had or done any work to the car recently which could be related?

To piggy-back on Sam's comment, a wiring diagram, or a service manual will help you to pinpoint where the wiring to the rear lights is run. And which color wire(s) you want to follow and look for issues.

If my memory serves me correctly, do the dash lights run through a rheostat which is connected or internal to the light switch?

This would explain why the dash lights go out when the light switch is taken out of the circuit.

It sounds like somewhere in the wiring harness, the back lights and license lights are back-feeding and receiving electricity from another power source which is always hot.

Because the lights stay on even with the light switch removed.

With the wiring diagram you can follow the wires from the back of the car moving forward to the fuse box and see if there is something possibly touching the rear lights wiring harness and sending power to the tail and license lights.

If it is the original harness, the wiring is over 40 years old and connections get loose, insulation can crack and rub though leaving bare wires a place to touch and and create problems.

From my experience, it's easier to replace either the one section of the harness and be done with it, versus splicing and repairing.

Elon

[PeteLeathersac]

To add to the above good advice...

Your mentioned ignition switch position has nothing to do with the lights being on or off...right?.

Is there a 4-way flasher system on these cars?.
I'm no Ford guy but a failed flasher module stuck in power-thru mode in combination w/ the 4-way switch in the on position and faulty or reversely installed bulb(s) is one combination that may cause these symptoms?.

Have you tried popping the fuses out one by one to see when the lights go out?.
If a fuse other than the tail/dash circuit causes them to go out, the circuit that specific fuse is labeled for may give you a clue where to look further..

W/ problems like these, sometimes a good visual and run through the fusebox then popping bulbs, disconnecting headlamps, dimmerswitchs, flashers and anything else in these circuits one by one may help lead to the source of troubles..

Good luck!.

~ Pete