You can see that portion of the electrical rough-in here.
Prior to mounting the camper I drilled a 1" hole in this tailgate channel stop to accept a 7-pin female 12VDC receptacle. Spliced into the wires feeding this left rear brake light to pick up the circuit. Also, robbed two of the pins inside the receptacle to serve as a feed and a switchleg from a switch up in the cab that would operate a brighter backup light on the back wall of the camper.
I mounted a agricultural harvester light with a trapezoidal pattern that would allow me to see the light in both side view mirrors. Mounted it a little high to distribute the pattern well and it works pretty good. Not sure if a flood would have worked better but it does provide ample light when backing up in dark spaces. Also have it switched inside the camper as well, in case I were to ever hear a rustle at night near the door.
It seemed that some of the manufactured units had upper brake light strips in combination with another pair of lights but most did not. My original plans were to have a 2.5 foot strip centered at the top in conjunction with the these 6" round lights. I had even purchased the strip but after the door was installed and the "much needed" rain gutter....something had to give. --> it was the light.
I considered height of install in respect to the 'seated approaching motorist' and wanted them in the plane of sight. These lights are about the same height above ground as the cab brake lights found on all modern pickups and I mounted them just above the vertical plane of my truck lights.
The closer proximity of the light arrangements are far enough apart to be seen as separate at 200-300' but close enough to grab one's attention.
The install didn't turn out half bad. I'm pleased with the overall setup, although I do wish I could have had somewhere to mount the upper brake light strip.