It's not that hard. Get a DPST switch which can be set up as on-off-on. Usually the center pin on the back is the output to the device/lights, or in your case the relay. The top pin can be for the reverse trigger, bottom for the highbeam trigger or whatever.
You need to tap into a wire that is hot with the high beams on. It is a good idea to fuse it. Run that wire to one of the pins on the switch as a power source. Do the same for the reverse lights.
Output from the switch goes to turn the relay on/off. Then the relay gets it's own power wire from the battery with a fuse or breaker, and the lights get powered from the relay.
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