Green wire: Coil negative. White wire: Dash lights(the ones the dimmer dims) Black wire: Chassis ground(any bare metal in the car) Red wire: 12 volts positive switched. That works on Autometer, Sun gauges, Autogauge and NAPA gauges. If a wire is marked "S" that is the signal wire( coil negative). You did not mention the car it is going on, be careful of some newer cars that operate with crank triggers and coil packs, really complicates the install. Remember, FUSE power leads and use grommets when going through metal. Christopher.