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What about a complete re wiring kit ...maybe get you ahead of the game if you have other/unknown wiring issues .
Agreed, I did my boat trailer last year new wiring was probably quicker than figuring out the problems. Tie string to end of old wires, loosen wires and pull it all out the front. Use string to pull new wiring back through.
Looks like a splitter that has the outlet plugs cut off & hardwired. Google "splitter for trailer wiring "
I would also recommend some dielectric grease when you put it together. Reduces corrosion so the connection is more reliable. Important in the salt.