[VetteVet]

This can be confusing especially if you jump around too much. Your meter is your best tool to check for voltage or continuity on a wire.
From what you said earlier about jumping the white wire to the green and yellow wires and getting brake lights when you pressed the pedal, you must have power on the white wire when the brake pedal is pressed.

If you put the red lead of the meter on the white wire and the black lead on a good ground. You should get 12 volts when you press the brake pedal. If you jump the white wire to the other side of the steering column connector to the white wire there, you can check the dark green and yellow wires for 12 volts when you press the brake pedal.You have to have the signal lever in the neutral position. If you get the 12 volts on both wires then your turn signal switch is ok and the problem is further down the line toward the rear of the truck.

If you think you're confused, I'm working with three other guys with similar problems. LOL.

Do your checks and read all the posts and you'll get it fixed. Let me know what readings you get on the wires and I'll let you know if they are correct.