Actually you don't have to rewire much to add the relay. Take the wire going to the switch and cut it(up front somewhere) and use it to control the relay. Run the wire from the lights(that you cut) to the relay then add a power wire from the battery to the relay. Whatever you have the switch wired to for power is okay since it won't be doing much other then control the relay.

Basically you are only adding ONE wire(relay to battery). Your existing wiring is fine.