This is on my '05 3500 4x4 5.9 Cummins...not sure how long this has been going on.
When I press the brake pedal, my running lights (cab marker roof lights, front cornering lights) come on. They are dimmer than normal but still very noticeable.

& I have a video below of what happens when I use my right turn signal (that function seems to be fine), left signal (NOT fine), & Hazards (also not fine).

I thought maybe the multifunction switch, but, that doesn't explain the running lights coming on when the brake pedal is pressed I don't think...

I've got no Check engine light coming on, no lamp out light, only light burned out on the truck is 1 tag light & it's been out for months...

Here's a video of the signals / hazards:

https://www.youtube.com/watch?v=xJL4...ature=youtu.be

 

Have a good look inside your trailer connector and its wiring. Take the connector apart and look into the back, where the wires hook up. Under the back bumper where the trailer connector lives, it's common for mud and gunk to pile up in there and short some circuits together, or for connections to come loose and short together. Also inspect the wiring leading to the trailer connector. It can easily become bare from gravel damage and can short together.

 

Thanks, I'll have a look back there.

 

I couldn't get the connector off for the round 7 pin plug. But I did have a look at the 4 wire flat connector that's been capped for as long as I've had the truck (almost 2 yrs.) & never used. It was full of mud & who knows what else, so I just cut the plug right off & no more problems! Lights work like they should again. (I did tape the wires up & put them back up as much as I could).
torquefan: Thank You!

 

You're welcome! Glad it was something easy.