I made my own bed rail lights for my 54 Dodge 20 years ago. I treated them as a 3rd brake (and turn) light on each side of the truck. My tail lights were in a panel below the tailgate. The lights that were available then didn't fit in the bed rail roll very well. I pulled additional wires off the brake/turn signal wires for each side. I drilled a small hole (big enough for the hot wire and a ground and a little clearance) at the top of the stake pocket and at the back bottom corner of the bed rail roll. I modified a plastic pipe plug that fit into the rail roll to hold a light. The light was brake light bright, and functioned with the brake lights and turn signals. A set screw in the end of the rail held the light "socket" in place. The method I used to fit the light in the plastic holder put the end of the bulb flush with the end of the plastic that was nearly flush with the end of the rail.

My 54 had a bit of a nose down rake, so sitting in a car that was behind the truck, the light in the rail roll was at a pretty high angle and provided very little additional lighting effect. It looked great if you were standing behind the truck though.

Pictures of the truck showing the lights in the bed rail roll are long gone. Gene