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I tapped into the reverse light wiring where it goes to the trailer plug behind the right tail light assembly. I used it as the trigger wire for a relay and also did the same for a second relay on the tail light wire, then wired them in series. That way the aux lights on my roofrack only come on when I have the headlights on and the shifter is in reverse.
So you're looking to add this hella light to work along side the OEM ones?
Super easy, if that's the case. Find the 12v reverse light in the trailer harness and splice in the hella light. You can grab a ground from harness or from the body.
The existing reverse light supply is good enough to power a couple of these. If I was going to run anything bigger I would use the reverse light supply to trigger a second relay that fed the lights from the 12v socket supply in the rear cargo area.
Light placement on roofrack
Aux lights are powered by the existing reverse light power, via a relay, tapping into the back of the trailer connector plug
Better view of the trailer connector tap
Tail lights are used to trigger a relay. This passes the power from the reverse light supply to the aux lights. Functionally this means that the aux lights only come on when both the tail lights and the reverse lights are on.