MasterBasher

Disclaimer: I have NOT verified this. I found the following in the Electrical manual I found at GM-Uplifter {Images moved to bottom so as not to bury the text} Looks like GN/BK (-) and GN (+) are the two you need for the Left Rear Speaker outputs. MM | color | Circuit | Description 0.5 | GN/BK | 116 | Left Rear Speaker Signal (-) 0.5 | GN | 199 | Left Rear Speaker Control (+) 0.5mm = 24awg 1.5mm= 15awg (approx) Based on my search in that document, and the location you are seeing it at, the larger gauge (1.5mm) BU/YE wire you are looking at _might_ be circuit 277-- Driver Power Seat Recline Motor Rearward Control BU/YE is used throughout the vehicle for many things, none I could find had anything to do with Audio. My suspicion is that due to the low wattage the head unit puts out, 24awg wire is all they used for the speakers. My opinion, not fact though...

Brought home my 2019 Silverado Custom Trail Boss today. Took the long way and put 160 KM (100 miles) on her to get home ;) best drive I've had in years!