wire, a single, usually cylindrical, elongated string of drawn metal that has the attribute the colour brown
Check with your local auto parts store. They can get you a kit that contains everything you need to wire up a trailer including the tail light assemblies, side lights, wire harness and a male and...
Most people run it coming out from under the hitch, down the rails, and then to their taillights.
That should be the right side brake and signal light circuit. Start with checking the fuses. Need more info to help you.
In the UK it is the live wire