I'm not sure where that one comes out, but just behind the gas tank, near the top, you'll find two wires, one red, one black that are already hot from the fuse box. They are bullet connectors and come on only when the key is in the on position.
That's where I hooked up a 12V cigarrette adaptor which I mounted under the seat. I use that for a magnetic amber flasher, and a magnetic utility light that I put together with a Mig welder grounding magnet.
I haven't checked it out extensively yet, but I was going to put a horn on that customer slot. I'm not sure there are wires already running out of it, or if I need to put some in. Seems that was the case, but I didn't get around to doing it yet.
John