If I'm reading it right, you have the G401 ground setup. Here's that one as well.

 

Instrument panel

 

I've read about LED Third Brake lights causing problems you are describing. Something about them going to ground. Think I read about one or two posts here. Unhook it and see?

 

Thanks for the replies and the schematics, i'll post back with my findings.

 

If it's wired directly into the reverse light harness in the back of the truck, I'd check there first

 

LOL @ Holty and schematics

 

I had the same problem with my turnsignals, my horn quit working too. I started searching grounds and found a lose ground on the driver side radiator support. Its up past the rubber mount facing the front of the vehicle. i cleaned it up and tightned it down and the trunsignals worked fine. so check that one out