A fused line to the switch, then out to the center pin on the back up light socket, depends on type of socket, some won't mater. Then ground the other side of the filament, could be the socket body or the the unused wire.
Positive from an upstream +12V. Second lead goes to my tail lights which then go to a ground for my lights. It doesn’t matter how you wire the switch, it’s just 2 pieces of metal that make contact when the gearbox is put into reverse. It doesn’t care if positive comes in through one wire or the other. If you hook it up as a ground path it’s still going to get +12V coming in one side and going out the other. Think of it as a piece of wire with a break in the middle. Put the box in reverse and the wire gets “fixed”. Current flow direction doesn’t matter in this case.