***EDIT*** ***2014 Express 1500, NOT 2016***
Hello, trying to help a friend here. 2014 Express 1500, he bought a trailer and a universal wire harness, hooked it up and worked perfectly for months UNTIL THIS HAPPENED...
He was pulling out (trailer wiring attached) of a steep driveway w/trailer and shredded the wiring (it was hanging down a bit) on the harness against the concrete. I'm sure that created some shorts and God knows what.
THE ISSUE NOW is , he has reconnected the Express factory configuration regarding where the universal trailer wire harness connected at the rear light clusters, and the following issues:
NO brake lights, only the center light works, NO turn signals, only the passenger front works and it is fast-flashing, Headlights FINE, Rear Running lamps when Headlights are on FINE, reverse lights FINE.
Emergency flasher switch works, you can hear it out of the driver speaker, but again only the pass.front turn signal is responding, fast-flashing. Pulled and checked all the fuses from BCM under the hood and 2nd BCM? (fuse central) under the drivers seat w/continuity meter, NONE ARE BLOWN. Not really sure how to check the relays, but switched a few similar ones w/ other similar ones to troubleshoot.
REPLACED ALL affected bulbs w/new ones (even tho they didnt appear to be bad), and no progress. Could it be the BCM(s) that are bad as I am SURE there must have been some serious shorting within the trailer harness wires that got shredded.
Anyway, we've spent enough time on this in the hot sun, and before he gives in and takes it to a electrical auto place, thought I would create an account here and ask. Thank you!

 

I would grab a wiring schematic from gmupfitter.com and start diagnosing from there