Not hard really, just takes time if you're mildly handy.
The best thing you can ever do to a boat trailer, especially one that is in salt is to remove the trailer from the electrical circuit. That is, run a separate ground from your trailer connector to each light. This requires buying lights that connections that are seperate from and do not use the stud as the ground. When you remove the trailer from the circuit, you get rid of the corrison issues assuming you've sealed up your connections and use LEDs. Also go with sealed LED's as they are less shock sensitive than standard glass blubs. I think it cost me right around $100 to do for the skiff trailer with some left over wire to do my jon boat trailer as well.
Most "kits" are too short by 2-4' and usually are the wrong pin-out. What I have done and recommend to everyone is this. Order a 100' roll of 4strand trailer wire, LED lights (back/brake & running), and a 100' roll of 16g wire in white (this will be your ground), heat shrink, marine grade butt-splices (Ancor brand) and a trailer connector that you can attach the wires into (now days the round 7-way are the way to go for most newer trucks).
When you pull out the old wiring, cut the wire from the front and pull the new wire back or cut the wires at the old lights, tie on
and tape tightly some strong nylon twine and pull the twine forward.
List of items:
Trailer wire
https://www.northerntool.com/shop/tools/product_1016438_1016438
ground wire (couldn't find the 16g link)
https://www.lowes.com/pd/Southwire-...White-Copper-THHN-Wire-By-the-Roll/1000777290
spare wire for clearance lights (hot side) buy the red version of the ground wire above (25' should do if you just have one on each side.
Connector
https://www.lowes.com/pd/Reese-Connector-7-Way-Plastic-Trailer-End/50437620
Brake lights (I think these were it)
https://www.agrisupply.com/trailer-light/p/84624/ https://www.agrisupply.com/trailer-light/p/84623/
splices
https://www.westmarine.com/buy/ancor--heat-shrink-butt-connectors--P009_275_004_003?recordNum=2
heat shrink 3/8" black will work
https://www.westmarine.com/buy/anco...ink-tubing-kits--P009_275_003_508?recordNum=1
Let me see if I can find my wiring diagrams I drew up.