Ed, you have a test lit fixture built into the system. Go to the front of the trailer, unplug the lights and plug the two plug/receptacle pieces in sideways 90 degrees to each other. Look at the wires into the rubber flat plugs, use the one with a brown wire on the truck and plug it in to the green wire corresponding pin on the trailer. If the parking lights are on the right tail light should glow red for brakes/turn signal. Now do the yellow wire and the left tail light should glow red.
Brown – Tail Lights
Yellow – Left Brake/Turn Signal
Green – Right Brake/ Turn Signal
Blue – Reverse Solenoid/Back Up Light(s)
White – Ground under coupler/ Safety Cable area