First you need to add the reverse switch and get power to it. Lot of the time people will buy a kit (with bracket and switch for the 4L80 or just hte switch for the 3-speed). You split power up at the horn, run it back to the switch, then back to the lights (I went along the main wire harness to the bad with my stuff).
Then I assume for this mod they modified the blackout light lense, and just removed the blackout light power and replaced it with the power from the backup switch.
(You go from the horn because the power is switched the same as it is for the rest of the lighting...)