Taillights fade and backup lights come on when brakes pressed

[leopold]

I am looking for some trouble shooting help. I was following the wife who was driving our 07 Yukon Denali and noticed that when she braked, it caused the tail lights to fade and the back up lights to fade up- not all the way to full bright, but enough to be concerning. She got pulled over later in the evening. I am wondering how to begin trouble shooting this?

The left turn signal may or may not be connected to this issue as it is blinking fast

 

[drakon543]

do you have a trailer connection? also if so does it use the adapter harness or is it spliced in. if its using the adapter harness you can unhook both sides and plug them into each other to remove the trailer wiring from the loop. simple free test and you can leave it unhooked for the duration of any other testing. another free test would be to remove the reverse bulbs to break the loop. regardless it sounds like you have a minor short between the brake/turn signal and the reverse lights. if the brake and turn signals work correctly with the reverse lights out (also assuming you replaced the potential bad bulb). using a multi meter you can connect a lead to either wire of the reverse lights and chassis ground checking for 12vdc. repeat for the other wire of the reverse lights. if you find 12v at the reverse lights with the brake pressed follow that wire until you find your problem. will probably be visible wire damage or fraying.

 

[OR VietVet]

Like @drakon543 said, sounds like a short or ground problem. Typically when a turn signal flashes fast, it means the other turn signal bulb on that side is out or shorted. Did you look at the front lights when looking at this happen?

 

[DRayson]

Verify the correct bulbs are installed in all tail light sockets. A dual filament where a single should be or vice versa can cause strange lighting issues