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Wiring A Boat Trailer For Brakes And LightsWiring a boat trailer is something that many boaters find themselves needing to do sooner or later. It may be to just connect the trailer lights on your vehicle, or you may find yourself rewiring the whole trailer with new LED boat trailer lights and a new wire harness. I have had light issues on an old trailer and to be honest it was quicker and way less hassle just to pull out the old wiring and lights and start again with a fresh wire harness and new lights. It is pretty simple to do, and then you know your lights will work!
I was doing a pre-season inspection of my trailer, which was only 2 years old at the time, and I noticed the ground wire on the trailer is screwed to galvanized metal, and the screw is not tight. A sure fire source of light problems! Make sure the ground wire on the trailer is attached to bare metal, not to painted or galvanized metal. Also make sure the ground connection on the light assemblies connects to bare metal. Many light assemblies use the mounting bolts as their ground connection, so clean the metal surface around the mounting bolt holes down to bare metal. Poor ground connection to the trailer frame is a common problem with lights that don't work. Below is a simple wire diagram for a standard four wire light harness. The white wire (ground) on the following boat trailer wiring diagram is drawn in grey so that you can see it against the white background. 
Notice the four prong connector on my boat trailer has 5 wires coming out the bottom. This is to simplify the wiring. There are two brown wires to provide power for tail lights to each side of the trailer. (one is brown and green and one is brown and yellow, the photo only shows the yellow side of the brown and yellow wire) When connecting the wire harness to the light assemblies at the rear of the trailer it is best to use waterproof Corroded wire connectors are a common source of trailer light problems, so better to do it right, and you will be trouble free for a long time. If you are wiring a boat trailer for boat trailer electric brakes the following boat trailer wiring diagram has two more circuits. One to control the boat trailer brakes, and one auxiliary power to charge the battery that you will need to have on the trailer so the brakes can be energized should the trailer come unhitched from the tow vehicle. Obviously you will need a six prong connector to connect the trailer to the tow vehicle in this situation.
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The standard four prong connector to connect the trailer to the tow vehicle is perfect for a simple trailer light system.
butt crimp connectors. They are available at marine hardware stores and are a crimp connector with a waterproofing adhesive inside and an integral heat shrink cover to seal the connection. You may as well get waterproof splice connectors to splice in the clearance lights while you are at it.