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Get a suitable relay and a switch, and wire them in directly. Grab power (do not forget the fuse) and put it on the contact of the relay, put a jumper to the coil so the relay can turn on. Connect your lights to the other contact pin so that when the relay is energized, the power flows through the contacts to the lights. Connect the other wire of the lights to ground -- if the lights are mounted to metal, then the mounting location is probably a suitable ground point. Finally, connect another wire to ground and to a contact on the switch, then another wire from the switch to the ground terminal on the relay.

Flip the switch to pass ground to the relay that already has voltage, so the ground causes it to energize.

Voltage on the relay contact pin will pass through the contact to the other relay contact pin and flow to the lights that are already grounded.

The ground from the switch will energize the relay, and the power from the relay will turn the lights on.

The pins on the relay are identified on the base of the relay.