I've been trying to diagnose a brake light issue for a few days. Here is what is happening

[Parking lights OFF, headlights OFF]

When pressing down on the brake
-Brake lights come on (both filaments)
-Parking lights Front/Rear - ON
-Interior Light above radio - ON
-Instrument Cluster - ON

[Parking lights ON (pulling the control knob one stop)]
-All parking lights work correctly except for the brake lights
-Brake light bulbs ON without pressing down the brake (both filaments)
-Pressing down the brake doesn't change anything


Currently I've been removing the wrapping on the tail lights wiring and looking it over. I did find out that each section has been cut at one point, and they used butt connectors to reattach everything. I've soldered/heat shrinked most of these connections. Eventually I will be removing all of the old butt connectors and rewrapping everything when I'm done.

Status of the other lights if you were wondering...
Headlights High/Low - No issues
Blinkers - No Issues
Hazards - No Issues
Reverse - No Issues ( need to replace a bulb I cracked when removing)

 

Sounds like you have the wrong bulbs installed or maybe not seated correctly. Going to take a guess and say that you have single filaments installed in a dual bulb holder. good luck

 

They are the correct bulbs, and I also purchased a new set just to make sure it wasn't the bulb itself. One of the issues is when I turn on the parking lights, the brake bulbs should only have one filament on, but it always has two.

Sounds like a grounding issue, but could this be related to the brake pedal switch?

 

Went back and reread your issues. I'm guessing this is for your 77? It sounds like your parking(front and rear), interior and instrument panel lights are turning on when you apply your brakes. It also looks like your parking lights when turned on, are connected to your brake lights (the reason why they are already on when brake is not applied. Try looking for splices that are joining them together. Those spliced/butt connectors might be your culprit. Someone must have wired them wrong. Before redoing those connections, make sure they are wired correctly. Someone here might have a wire diagram you can refer to.

 

Since you already went over the splices, check if the wires at the bulb sockets are melted together in the tail lights.

 

I didn't have a lot of time to check on it today, but how do you remove the base of the socket to check the connections behind it? I pushed on the rubber grommet from the back of the tail light assembly a little.

 

Honestly not sure but id assume the grommet needs to be pulled out, or the whole thing needs to be pushed thru.

*EDIT* better yet, disconnect the tail light and use a DMM and check for continuity between the brake light and parking light socket. On both lights

 

You get access to the back of the taillights by taking off the panel in the bed.

 

After unwrapping the harness in 5-6 different places, I've figured it out. There was a section of wire (green & green/white ) that appeared to be touching which would make sense. The harness had been completed cut in 4 separate places and butt connectors were used to reattach everything. I took the time to remove those, and solder/heat shrink all the connections.

Thanks for the help