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Is the cab/third brake light inop also?Do you have a digital multi meter or test light to do a few electrical tests?
The problem may be caused by the turn signal/wiper switch we call the multifunction switch. But lets do a couple electrical checks to make sure. First go to the brake light switch and check for power solid light green wire with the switch plugged and step on and off the brake pedal. See if that wire has power when you step on the brake pedal.
Ok. You will need to take the trim panels off the steering column and check the wiring connectors at the turn signal switch/wiper switch. We have seen the wiring pull loose from the connector for the switch. Check for power on the LIGHT GREEN wire at the turn signal switch. The power from the left and right brake lights goes through the switch and the power for the third brake light goes straight from the brake light switch to the third brake light. You may want to check the bulbs in the third brake light.
Make sure you are on the correct wire. Below is a picture of the connector. The wire is in pin #511.
Unplug the flasher relay. It is in a black relay box under the dash. You should not have power on those two wires unless the turn signals are on.
Do you have a trailer harness on the rear of the vehicle?
Is it factory or added on?
You may want to inspect the wiring. You should not have power on pin #2 with the relay removed.
Wait. Do you still have power on pin #2 with the turn signal switch unplugged?
Check the right front turn signal bulb and socket. Pin #2 goes to the right front turn signal. You have a short to power on the right front.
Just reply back on this post when you are ready. I will be on line tomorrow afternoon.
#511 gets power straight from the brake light switch when the brake pedal is depressed. And since you have power at the switch, the wire is broken between the brake switch and the multifunction switch. But I wonder what else is wired into the light circuit that would cause the draw and cause the lights to dim. Either a large load or a short with high resistance causing this problem.
Try that. Do no splice it is, just run a jumper from the brake switch to the turn signal switch. Also, disconnect the brake controller and look under the dash for any strange wiring splices.
So all you did was unplug the controller?
Follow the harness down the column and make sure it is not pinched between the column and the metal dash brace.
The harness may be cut somewhere down the steering column.
Are you still getting power on pin #2 at the turn signal switch?Also check pins #5 an #9.
There is something shorting the system out. You will need to go under the vehicle and follow the harness to the rear tail lights.
This may end up requiring a hands on electrical mechanic to find the problem. But keep me informed.