Factory wiring setup for tail lights

To be like new you need a chassis control module on the frame behind the transmission. Fuses to power that up behind the LF tire. That connects to the cab control module. The turn signal switch wires are the size of old school phone wires. Tiny they are. The fuse for the LR also covers the RF and vise versa. Simple!
I would just mount a good rectangular junction box at the best place you choose. When I updated some old heavy haul trailers I found custom harnesses were available from Truck Light. They were expensive and the delivery time was long. Used their round junction boxes but they would look crappy on your truck.
Bought harnesses with 2, 4 and 6 wires in them for the back. Used heat shrink connectors but a friend of mine told me I am cheap. Connections should be soldered. I never had a problem with them properly crimped and sealed. They worked well even backing trailers into the ocean for cranes to unload bridge beams.
I would see if you can order harness sections that plug into the lights and would make it to your junction box. That should be easy and would look sharp.

 

Pretty sure you can order it any way you like. Cab and chassis come with the old signal stats bolted on the frame but all wiring is sealed. They also come with a BOC junction box at the trailer wiring for body builder tie in. Its nothing special, just your everyday plastic junction box. Most newer tractors are all factory sealed with LEDs inside the rear crossmember for a nice clean look, plus they are out of the way.

 

Every manufacturer has a body upfitters manual, most are online.

they explain exactly what you are looking for.

 

You forgot to add run wiring tight as possible, make sure harness has plenty of pinch points, and cut ties at sharp razor angle! Some of the better sealed wiring I worked on was on fuel tankers that had to be spark resistant. Had compression style fittings at the junction boxes. Overkill for you. You plan on lots of rear lights or just the minimum required?