No brake lights

If I remember right, there should be 12 volts on the brake light switch orange wire all the time and on the white wire when the pedal is pressed. If the 12 volts is missing on the orange wire, check the fuses. If you have it there but not on the white wire, replace the switch.

Need to know the model so I can dig up the right wiring diagram.

I looked at a 1992 GMC Jimmy. The orange wire leaves the stop light hazard fuse on an orange wire, but they show a pink, white wire going to the brake light switch and a purple wire leaving it. Regardless of the different colors, the only thing I can think of that is in between is the firewall connector. You are going to have to back probe both sides to see if there is a corroded connection in there. If your vehicle shares the hazard circuit with the brake light fuse, those wont work either.

It is indeed an orange and a white wire for this circuit.

So you found 12 volts on the fuse but not at the switch. Be aware there are two different fuses labelled "Brakes" and "Stop / Hazard". To complicate matters even more, no connector terminals are shown so I can not find good test points for you.

The "Brake" fuse should only have 12 volts when the ignition switch is in the "run" position. That is for the Anti-lock Brake Controller and is not related to the brake lights. Double-check the voltage on both sides of the "stop / hazard" fuse. Next the orange wire leaves the fuse box, then four wires are sliced onto that one wire. Those feed the brake light switch, the "RWAL" module, if you have rear-wheel anti-lock brakes, the hazard switch, and the audio alarm module. I can not tell from the diagram which side of the firewall that splice is on, but we can figure it out. It is pretty likely if there is a corroded connection in the firewall connector, it is going to involve just one terminal, not four individual circuits. Therefore, If the hazard flashers also do not work, it suggests the splice is inside the truck cab and the one bad terminal makes all four circuits dead. If the hazard flashers DO work, the break in the brake light circuit has to be on its own circuit, meaning four separate wires go through the firewall connector and the splice is under the hood.

If you can find the hazard flasher, you can check on its orange wire for 12 volts. If you do not know where it is, that voltage should also be on the brown wire at the turn signal switch. There are two flashers. The turn signal flasher has three wires. The hazard flasher has only two wires. See how far you get with those voltage measurements.