Trailer Lights Flat Plug Question - Help Needed

My little 1996 Aluminum Ranger Cherokee had some trailer light issues so I fixed and replaced some things in the lighting system and cleaned it up. While doing this I saw the wires on the trailer wiring plug were cracking and brittle so I replaced the trailer flat plug. It is flat and has 3 male naked prongs and one insulated prong. Typical flat plug wiring. BUT coming into the old plug was 5 wires not 4 wires???? I found 4 typical wires and one larger black wire. I ohmed the wiring out with an ohm meter and found the larger black wire internally tied to the brown wire inside the plastic plug.

Does anybody know what the deal is with this black wire and why it is bigger than the rest of the wires??? There is no surge brake or anything like that - just a typical little aluminum boat and trailer - although it is a Ranger-Trail trailer made by Ranger.

The black wire was bigger than a typical trailer light wire. What I did was tie the black trailer wire and the brown trailer wire together and tie them both to the brown wire on the new trailer male plug.......OK or not????

Thanks.

Panama

You're good. They often split the brown for the two sides of the trailer. Don't know as I've ever seen one black.