Hi David,
I'm really sorry for the mix-up and for how hard it's been to get ahold of us — that's frustrating and not the experience we want you to have. We're actually open every day, Monday–Friday, 9am–6pm, and you can reach us directly at 615-610-1413.
I went ahead and checked our email and phone systems on my end, and I'm not finding any messages, return requests, or calls from you. I'd like to make sure nothing slipped through the cracks, so please don't hesitate to reach out and we'll take care of you.
In the meantime, for your vehicle you selected, 2024 GMC Sierra HD Crew Cab with BOSE, the harness plugs into the top plug of the factory amplifier. This video shows how to access the amp so you can unplug and re-plug it:
https://www.youtube.com/watch?v=D4ha8UVC-mo
Don