Tap into the +12V supply to the side lamp if you want an ignition switched feed and don’t forget to fit a suitably rated in line fuse. That would be the quick fix.

However, it would probably be better in the long run if you tapped into one of a supply to either the tail light or rear brake light switch to trigger a fused relay mounted under the seat somewhere; then the load can be controlled by the relay/ignition and drawn straight from the battery. You’ve also then got a decent, meaty, high current ignition switched supply for other accessories you may wish to fit later.

BTW - Welcome to XT660.com!

__________________