FD Harden

I have a 2015 gmc sierra 2500 denali I have had issues with the volume going up by itself when it happens the volume indicator shows up on the dash instrument cluster like when you use the steering wheel controls. I have had it into the dealership twice for this issue they did a software up and the issue is still there also checked the volume control buttons on the steering wheel.I also turned the auto volume control feature off to eliminate that as a possible cause. Has anyone else had this issue. I am also having issues trying to locate a wiring diagram or electrical schematic. Any possible ideas would be appreciated thanks.