I am looking at the wiring schematics for a cab and chassis. I assume that is how the vehicle was ordered before the flatbed was installed. If that is incorrect, let me know.
Now, what you described is an open circuit. The signal and brake light power from the same source, so it could be a power or ground issue. What really needs to be done is when it is happening, you need to check for power to the white wire with the light green tracer (see pic 1). If there is power, then we know it is likely a ground issue. If that is the case, simply try running a new ground from the light to a known good ground location such as the frame.
The black wire in the pic which is the ground is actually located (mounted) in the engine compartment (driver's side rear of the compartment). Note that the ground for the opposite has a different ground location, so it may be a good idea to check the condition of the ground, make sure it isn't damaged, corroded, or partially broken.
Are you able to check it when the problem is happening? Also, if the body company used a different light assembly than the vehicle came with, open the lens and wiggle the light around to make sure it isn't a bad socket.
Let me know if this helps or if you have other questions.
Friday, November 20th, 2020 AT 8:42 PM