Again, you're ordering from Amazon so I presume it's an Auxbow. If so, it comes with a relay. Of the wires extending from the relay, there is a larger (12 gauge red wire) with a 20 amp inline fuse that connects directly to the battery. In fact that relay needs to be mounted adjacent to the battery. Positive to the rocker will be switched power from your subharness. If you don't use the supplied switch and you buy a rocker switch from Amazon, be sure to jumper power on the switch to the pins for the two led's in the switch. The Auxbeam does come with instructions.