You should be able to install a relay at the rear of the flat bed. Use the current taillight feed from the front to power the low side (86) of the relay. Ground wire from low side (85) to a clean spot on the frame. Then you'll just need to run a fused power wire and correct gauge wiring from B+ to the high side (87) of the relay. Then take the high output (30) from relay and use it to feed all the led marker/tail lights you want. Lol If you need more help when the time comes, i might be able to help. I might've used ta kinda do that stuff.
old thread but I installed peterson lights on my 86 and did not need to add the "led" flasher to get them to work. Some don't need it, it depends on what you choose. normal heavy duty mechanical flasher worked for me. All lights in the rear are led's my front are still incandescent. Except for the cab/chicken lights
