You have a few choices. In my case, I had to use an additional 4 pin plug to run the Air Force One. One of those pins were dedicated to the light, so I was OK. If you are not using any of the pins in the 7 pin connector, you can use them. A choice would be the brake line (5 o'clock in the RV socket) if you're not using it for the SMI, or any other pin you are not using. Backup ( center pin), or even +12V (1 o'clock). You might have to clip some wires if you aren't using that feature for anything, but if the brake pin is open, that line goes all the way to the RV dash.
Because I used an LED on the dash I used a 16ga wire. It was just a handy size I had. LED's don't draw much power so I'd think anything would work.