If you have a "universal" Chevy Truck harness that fits many years, it is possible that the extra wire is for a back-up light or lights. Chevy keeps some things, like color codes, forever, so this MAY be correct for you, Brown = running lights, rear, DK Green = brake/turn right side, Yellow = brake/turn Left side and Light Green = Back- up. I hope this helps.
best wishes,
Les
