Trailer electrical wiring and harness plugs replace worn parts in vehicle electrical connections. The most basic design is the color-coded 4-pin system that uses a brown wire for tail, license, and side marker lights, yellow wire for left hand stop and turn, green wire for right hand stop and turn, and white wire for system ground. 6-pin connectors have one terminal each for electric brake controls and 12-volt accessories, in addition to connections for turn signals, brake lights, taillights and ground. 6-pin connectors come in square and round styles and are common to gooseneck and fifth wheel trailers, boat trailers and campers. 7-pin connectors are commonly found on new vehicles that are prewired for towing.