Items you will need
Trailer wiring kit for trailer with electric brakes
Trailer break away switch kit for trailers with electric brakes
Solderless butt connectors
Solderless ring terminal
Self tapping screw
Nylon wire ties
Solderless terminal crimping tool
Electric drill with socket to fit self tapping screw
Wiring a horse trailer is a project that will likely take from one to three hours for someone working alone. The task requires only simple skills that, if not already known, can be learned quickly while completing the project. No expensive tools are required, though you will need a solderless terminal crimping tool, wire cutters and a pair of pliers. An electric drill equipped with a socket will make installing the ground screw easier also.
Tape or wire tie the wiring harness from the trailer light kit to the trailer tongue. You want the trailer connector to extend past the trailer coupler by about 1 foot. This is a temporary step to ensure that the harness doesn't get pulled too far in to the trailer frame during the wiring process. The tape will be removed when the wiring is done.
Separate all but 5 feet of the white wire from the wiring harness. Cut the white wire off at this point (5 feet from the trailer connector). Strip 1/4 inch of the insulation from the end of the white wire and insert in the solderless ring terminal and crimp the terminal to the wire using a crimping pliers. Attach this ring terminal to the trailer frame with a self tapping screw after removing the paint from the trailer frame where the terminal will make contact.
Separate the blue wire from the trailer wiring harness. Run this wire down the left trailer frame rail and connect to one lead of the electric brake, and then continue across the axle and connect to one lead of the other trailer brake. Connect the other lead on each brake to the trailer frame using a solderless ring terminal and self tapping screw.
Mount the break away safety switch on the trailer tongue with a bolt and nut or a large self-tapping screw. Mount the switch in a location which allows the actuator cable to reach the vehicle trailer hitch.
Splice one lead of the breakaway switch to the blue brake wire using a Scotchlok connector.
Connect the second lead of the breakaway switch to the positive terminal of the on-board trailer battery.
Connect the negative terminal of the on-trailer battery to the trailer frame using a self tapping screw and a ring terminal.
Separate the green wire from the wiring harness and run it down the right frame rail of the trailer. Connect this wire to the right turn/stop light using a solderless butt connector.
Run the yellow and brown wires down the left side frame rail of the trailer. Connect the yellow wire to the left turn/stop light using a solderless butt connector.
Connect the brown wire to the tail lights. Use a Scotchlok connector at the side marker light to connect that wire. Use a second Scotchlok connector at the same location to attach a wire to run across the trailer to the side marker light on the other side. Connect that marker light with a butt connector. At the left tail light use a Scotchlok connector to connect the light and continue the brown wire to the right tail light. Connect that light with a solderless butt connector.
Use a Scotchlok connector to connect the wire from the triple bar light in the rear center of your trailer if the trailer is so equipped.
Tape all connections with electrical tape and secure all wires to the trailer frame with nylon wire ties.
Remove the tape holding the wiring harness to the trailer tongue.