So I havent completed this swap on my wife's 2018 LT, BUT, I believe I have all the components. The factory engine to body harness will have the Brown/Violet wire already in it. I was going to build my own harness and pin out this connector on the other side, but for $50 I got a factory GM harness that is plug and play with turn and fog connectors.
For inside the car I got a factory headlight switch with the fog button. I'll pin out the empty hole and run a wire to the BCM connector for the fog lights and (fingers crossed) voila. I believe you only need the BCM reprogrammed so the indicator like on the dash illuminates, but we'll find out.
Did something similar with my 2019 Silverado with the factory off road light button.