While I was converting this old boat trailer to a utility trailer, I noticed I needed to redo the lights because the old ones were cracked and non-functional. I started by getting a trailer light kit at Tractor Supply for $17 which included the wiring harness, two rear turn signals, two side mounted lights, and various connectors.
Step 1: Snaking the New Wire Through the Frame
I started by using one of the many connectors to connect the new wire to the old wire. I then pulled the old wire out while pulling the new one through.
Step 2: Connecting the New Lights
After I attached the new light, I took off the front cover so I could attach the wires. After reassembling the rear light, I moved on to the mid side-light. I started by using a brush on my grinder to remove all the corrosion from the plate that the light attaches to. The middle light relies on a good ground to work so it is necessary to get down to bare metal. I then attached the light, pulling the little black wire next to the dual wires going to the back light. My kit included these little clips that pretty much allows you to connect the wire coming from the side light to the main wire going to the rear lights. To use them, you just put the wire you want to connect to in the clip, then put the wire you're giving power to in the clip then you crimp down on this metal tab. After that, all there is to do is fold this little piece of plastic over the whole assembly, creating a quite nice looking connection. I then used the same procedures to wire the whole other side.