It wouldn't be too hard, they're easy to find, inexpensive, and not big or heavy.
Run something like 12 gauge wire so it will support a large load should the need arise, and fuse it at the battery.
Be sure to use a rubber grommet in the hole where you run it thru the firewall (to prevent rubbing thru the insulation and shorting out) or wrap a couple of layers of electrical tape around the wire there.
Hardest part is probably running the wire thru the vehicle.