Guys, I'm thinking about buying a 2014 GMC truck. On the left inside of the bed is a black box mounted to the rear side post near the tailgate. It has wires coming out of it. I may be an idiot, but I can't figure out what it is. Truck has a topper with LED lights mounted under the bed cap, but I don't think that has anything to do with the box? Could it be some kind of electrical box for trailer brakes, or what? Any ideas? Wish I had taken a pic.