Third brake light not working?

I'm having a temporary glitch right now that won't let me in to our AllData service manuals, but I'd still like to work with you since electrical is one of my specialties. You're miles ahead of where most people would be in the diagnosis, but allow me to suggest a few ideas for now until I can look at a diagram.

The first thing I noticed is you're doing a lot of continuity checks. Those can identify a solid break, but often not one with high resistance. Next, putting 12 volts to a point in the circuit can tell you what works, but it takes a lot of work to do that. I mean dragging wires around and looking for good ground points that typically have a lot of paint on them.

My preferred method is to find where the circuit is getting 12 volts and following that until it is lost, but that has to be done with a load on that circuit. The biggest mistake I see that leads people down the wrong path is not having that load, or complete circuit.

I have a real good example of that. Back in the mid '90s at the dealership, a coworker asked me for help on an overheating Dodge K-car. He found the radiator fan motor was dead. After unplugging it, he used a scanner to activate the fan relay, and found 12 volts feeding the fan was pulsing on and off, as it was supposed to be doing with the scanner. The obvious solution was to replace the fan motor.

A few days later when the motor was installed, it was still dead, but he had the 12 volts going to it. He had checked the ground wire already. I asked if he was finding that 12 volts with a digital meter? "Yes". I told him to use a cheap incandescent test light instead. He looked at me like I was stupid, but a few minutes later he was back. No 12 volts with the test light.

To boil this sad story down, the old motor had developed tight bearings so it was drawing more and more current until the day it burned open the fuse link wire. Think of them as a slow-blow fuse. When they burn open, the last step is arcing inside the insulation. That leaves a residue of carbon behind. That carbon was more than enough for a tiny tickle of current to get through for the meter to falsely see 12 volts. Had he measured that while the motor was plugged in, he would have found 0 volts. With the test light, it provided the load and not nearly enough current could get through that carbon track to make it light up. This is where the cheap test light is more accurate than the expensive voltmeter.

My coworker was correct to replace the tight fan motor, but the rest of the repair was to solder in a new fuse link wire.

Now that I shred that wondrous story, see if you can take voltage readings in that circuit with everything plugged in. The bulbs will provide the load that makes a poor connection become apparent. If the connectors are sealed and you have to unplug them, use the test light instead of the meter. Continuity tests have the same shortcoming. The current they supply in the Ohms test is so small, all it would take is one tiny strand of wire still intact to get a reading of 0 ohms, but you wouldn't get enough current through that to run the bulbs.

Next, where are you finding those black / white and brown wires? Could you have their roles reversed? Typically the black / white would be a ground wire. Regardless, what I'm leading up to is GM is somewhat famous for running a wire harness under the carpet under the driver's foot area. That's a place where a lot of people find wiring problems, but to my knowledge, there's no connectors under there. If you're applying the 12 volts further back, you might consider looking under the carpet for wires that are rubbed through and / or corroded.

I'll be able to do a better job when I can look at a diagram. When that happens, I can post the diagram for you if you need it.

Okay, here's the diagram for your truck. We know everything is okay up to splice S100, (blue arrow), because the other brake lights are working. Connector C320 is in the middle and might be a good place to find test points and corrosion. They don't show its location, but I found this chart that lists it as being above the rear of the headliner.

In the last diagram, I blew up just the left half with the center brake lights. The black / white is the ground wire, and the light blue you found with the missing 12 volts is the 12-volt feed. The ground wire goes through C320 too, but it is shared with the cargo lamps. If those lamps work, the ground wire has to be okay.

Given the missing 12 volts, the next best suspect would be a corroded splice, S100. That is shown in the fifth image. It appears to be a part of the harness that runs along the bottom edge of the dash, right above the accelerator pedal.

These can be a little difficult to read. You can copy them into a typing program and expand them there. I use MS Word. I expanded the splice location a little in the last image.

See how far you get now and let me know what you find.