Just use twin core cable? That’s what I have done for all of mine. Rear work lights, roof lights, bullbar lights. All have the power supplied by twin core so the earth is the. The same one the relays use to work. Chances are wherever you have got your power supply and relays will have a far better earth than the end point where the light is.
I took it a step further and my earth is the bus bar at the top of the auxiliary fuse board that supplies everything with power. So I have fuse box and relay box in the battery compartment next to each other. With all weathers for everything additional also back to the battery
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