Go to boatwireusa.com and order 1 each 100' colored wire and build your own harness. Order enough heat shrink butt slice connectors, they will seal up when you hit them with heat. You'll have to have splices for side marker lamps, trailer lights, brakes, etc....
Junction box in the front.
Junction box in the back.
Get the correct wire and wore that in. Then just run each light wire seperate out from those Junction boxes.. you should never have to run the Junction box or main wire front to back again. Just trouble shoot and run the individual wires and plug whip wire as needed.
Sometimes a third Junction box is needed but that main wire front to back is the base of the project.
Grem is your man if you want an expert job. He has performed the work for many and helped even more. Just do a search on trailer lights and you’ll find many great threads on trailer lights.
If you don’t want to search, one common solution is, buy rolls of marine tinned wire from a boating place, buy heat shrink self-sealing connectors, proper wire strippers, and a very good crimping tool. Buy a low to the ground mechanics seat with wheels so that you’re not on your knees and back. The job will take about 6-8 hours if it’s your first time.
I rewired my trailer over 3 years ago. I do not have guide posts on my trailer, so all of my connections at the rear of the trailer are subjected to under water submersion every trip. My focus was to make all crimps as solid as possible and ensure that all connections were properly heat sealed. Since rewiring, I’ve had only one wire fail but I diagnosed it quickly and patched the issue with new wire. I fault myself for making a bad crimp and improper heat seal on that connection. Other than that 1 failure, the junction box solution works great and I did not have to add guide posts.
Often the side running lights are spliced into the same wire that runs to the tail light. Splices are a point of failure. I ran independent wires to each light. No splicing.
Take your time and focus on the crimps and heat sealing. Use marine tinned wire. If you want to do extra, encase your long runs of wire in protective wire loom, which minimizes rubbing and abrasion. Overall, the junction box, independent wires, and self sealing connections are the ticket.