I've been working on installing a backup camera to mount above the rear hatch on my camper shell. This will be 6 feet above the pavement & look back (and hopefully over the tops of any small trailers I might haul).
The camera only draws 300 milliamps at worst, so I thought I could tap 12 VDC off the backup light. I removed the right rear light & found wiring to the bulbs easily accessible.
I drilled two 3/8" holes side by side in the bulkhead just forward of the tail light compartment & joined them by hammering an inch-wide metal chisel through the metal between the holes to make one huge slot in the bulkhead. This will be invisible after the rear light is reinstalled. Access to this bulkhead with a power drill was easy.
I drilled a single 3/8" hole through my bed liner & through the inner metal of the bed a couple of inches ahead of where I estimated the bulkhead was. This hole was about 6" above the floor of the truck's bed. If I had removed the bed liner before drilling I could have done a neater job, but didn't care to put that effort in.
The two holes were only a few inches apart, but running an electric wire between the holes was difficult, until I learned that the space ahead of the tail light assembly, between the bed's inner and outer walls, was completely open to the ground. I taped my electric wire to the end of a 3 feet long piece of 14-gauge galvanized wire, pushed it through the bulkhead hole I had made, and after a few tries, saw the taped ends of the wires hanging out near the tailpipe. I removed the taping, pulled the stiff guide wire back, leaving the electric wire hanging. Then I pushed the guide wire through the hole in the pickup bed. After a few tries its end could be seen hanging out near the tail pipe. I taped the electric wire to the guide wire & retracted it into the truck bed.
While I was at it, I ran another length of electric wire through the same gap, in the thought I might install a high-mounted 'stop' light above the camper shell gate. Both these new wires will run up the wall of my camper shell towards its roof.
This saved me a lot of crawling around under my F150.