and a ground from that section screwed tight to trailer frame and same for right side.
What Kind of lights are you using..? Usually the lights ground thru the mounting bolts.... and the wires are for power..
Green is right (stop/turn)
Yellow is Left (stop/turn)
Brown is both (clearance)
White on the 4 way plug is ground to frame..
Unless you have the rubber mounted sealed units, then they supply a white wire for grounding to frame..
Make sure wires or bolts go to "shiney metal" as mentioned..
Also if trailer is a "Tilt" type....make sure ground is continuous thru hinge point (dissassemble and grind "shiney" or add a jumper for ground..)