Having the same issue on my 2017 Denali 2500HD. Tried turning the auto lights off and turning on manually but that didn't work. Currently only the right trailer turn signal does not work. Checked the fuse and it's good. Tested for voltage at the fuse and nothing. Turning on the left turn signal I can hear a click like a relay is being engaged and disengaged as the light blinks but do not hear that when the right signal is on. Thinking it's the 'engine compartment fuse block'. No signs of melting though. Thankfully I have an extended warranty and that is covered. Will post once I have it diagnosed and fixed. Will be a few weeks.
