Unfortunately the cheap, off the shelf, one size fits all trailer wiring and lighting kits do not last on saltwater boat trailers. The most important things you should consider when choosing lights and rewiring your trailer are:
1. Marine grade wire
2. Single point ground
3. Potted circuitry LED lights (I would make sure the lights you purchase are made using potted technology. This process encases the circuit board with resin to completely seal it from moisture. We use TecNiq LED lights exclusively in our trailer wiring and lighting kits. TecNiq also makes underwater transom lights made with the same potting process. Knowing that their underwater lights hold up being constantly submerged on a boat, I feel that they are the best option for boat trailer lights.)
Kraken Wiring designs heavy duty wiring and lighting harnesses specifically for salt water boat trailers. If you want to build your own trailer wiring and lighting harness versus buying our kit, I would recommend the below method. This is the best way to bulletproof your boat trailer wiring and lights DIY.
1. Home run the 16/3 triplex from both tail lights all the way to the junction box. Do not ground your tail lights with the mounting hardware to the frame at the light. This is the reason for 16/3 triplex. Splice in all wires from the tail light using waterproof butt connectors. Then use adhesive lined heat shrink over butt connectors.
2. Run the 16/2 to the side marker/clearance lights and splice in your hot and ground wire with waterproof butt connectors. Again use adhesive lined heat shrink over butt connectors. This helps to protect the butt connector from UV rays. If you don't put heat shrink over the butt connectors they will become very brittle from UV. Then they will crack, water enters the connection and the rest is history.
This is the junction box & 7 pin harness. https://www.amazon.com/Wadoy-Trailer-Junction-Connectors-Vehicle/dp/B0759J3WWT
Triplex wire: 16/3 AWG Marine Grade Wire Triplex AC, Tinned Copper USA Made Boat Wiring
Duplex wire: 16/2 AWG Marine Grade Wire Tinned Copper USA Made Boat Wiring
Butt connectors https://www.jamestowndistributors.com/userportal/show_product.do?pid=706
Use die electric grease on all plug and play connectors and inside the junction box.
DO NOT ground every light to the frame using the lights mounting hardware.
Always run a dedicated ground wire for each light all the way to the vehicle connector or junction box. Then run 1 ground wire and attach it to the frame near the tongue where it will not get submerged when you launch your boat. This is overkill to run another ground to the trailer frame, but important if the grounding in your actual vehicle is potentially faulty.
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