In a five wire set up you will need to figure out what each wire from vehicle is being used for. ie Brake and Tail lamps R & L, Turn signals R & L and ground. The connector from the trailer will correlate to the vehicle. The other two empty slots on a 7 way are for adding a Trailer brake controller. In short, when plugged together the wires that control trailer lights should match up to the vehicle lights wires. A test light and a buddy are all you should need to figure out which wire is to what circuit. Turn key on, Have buddy hold brake pedal and put test light on each pin until you find power. and so for turn signal and reverse lights if equipped. Black is usually ground but could be green