Old Man with a Hat
Jul 4, 2012
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Central Oklahoma
This is on my '69 D300 (hey, electrons are electrons!). I wired up new taillights on my truck not long after I got it. Recently, my project became streetable, and this is when I found out I had rear lighting problems.

The stock wiring harness is intact and uncut. Using the FSM for the truck, I ran the lights to the correct wires in the harness, and initially had all but brake lights. Found out the factory brake light switch plunger was busted, so I replaced that switch. Now, all of the taillight functions were working. Great thing!

Now, the brake lights and running lights in the rear ONLY operate apparently when they feel like it. My turn signals and hazards work fine, and have not failed to do so. The brake lights and taillights work on a purely random basis (when the circuits are applied, it's really a 50/50 shot at function!). I am using normal incadescent bulbs throughout - no LED or weird aftermarket glow lights.

The wires are fine, and the brake switch is new and correct. The taillights are basically trailer light assemblies that are fully functional. Fuses are fine, as the lights DO work when they decide to. All of the front lights work upon application - headlights hi/lo, grille and fender turnsignals and the grille parking lights.

I've gone back and looked at the wiring connections at the lights, as that is the only place I can think where there is (or could be) a problem. No change.

So...ideas? Where am I missing the cause?
It's probably that simple, as I think about this. I had bolted the lights to the steel bar they are mounted on, after I had painted that bar. A wire wheel to the mounting surfaces and a check of the are probably dead-on, John. I'll have time tomorrow evening to work on that. Thanks.
Grab you jumper cables and hook one end to the tail light assembly and the other end to a good ground. If a bad ground is the issue, this will cause the lights to function normally.

If the ground isn't the issue maybe this will help. I have a 71 D200 that I have experienced similar problems with. The culprit on mine was the main harness connection through the firewall, just outboard of the master cyl. I had to unplug it and clean the contacts a couple times from the under hood side and shot a little wd 40 on the connectors to get mine working consistently. hope this helps. Pat
Well, they're working! It was a bad ground where the lights were attached to their mount points. Took a wire wheel and cleaned up all four point to where they'd make good ground. The paint I'd used prevented the ground from occurring.

To ensure thoroughness, I also cleaned the bulkhead connectors with contact cleaner. Bonus there was now the overhead cab lights are now working. Patience and logic. Thanks for the help, guys!
Like the saying about 90% of carburetor problems are ignition related. Same for lights, almost always a ground, if they do anything odd, definitely a ground.
It sounds like something is wrong with your tail light, not the car's electric system. I had a similar situation with the LED headlights I bought from Vont Smart Car Light Strip. I installed them on my own, and they were wrong because they won't switch on. I went to a car repair shop, and they said I didn't connect the cables correctly, so it created a short circuit in the system, and the Headlights won't turn on. Luckily I went to a car repair shop because it could transform into serious problems with the car's electric system.
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