It has been a while since I've looked in that area but as I recall, you need to remove the inner panel on the lower tailgate. Pop out the middle piece carefully and unscrew the other piece. You should be able to locate a brown wire, which I think is the reverse light wire. (As I said, it's been a while.)

I'm guessing that you want to tap into the reverse lights so that your reverse camera will know when to be activated. That brown wire, or whatever color it actually is, should do it.

Good luck!

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