I was going to copy this over from R-F where I originally posted it but....
This how-to has wiring diagrams for three different ways to control auxiliary lighting devices. Typically this fog lights, driving lights, off-road lights, or back-up lights. The concept can be applied to anything.
You should always use but connectors and insulated spade connectors for your connections. Do not ever use household wiring type wire nuts to twist wires together. Fuses and proper wire gauge are very important, cheaping out on this step could set your truck on fire.
#1: Controlling lights with a switch
The lights are turned on and off only by a simple switch in the cab. This is the most common. I recommend getting power for the switch from a circuit that is only on when the ignition is on. That way the lights will never be left on while the engine is shut off killing your battery.
#2: Controlling the lights with an existing circuit such as high beams or reverse lights.
This will slave your new lights to the existing lights. I do this all the time with my off-road lights so they come on and off with the high beams. That way I don't have to push buttons to turn them on and off. This is also what you would do with reverse lights.
#3: Controlling the lights with both a switch and an existing circuit (on-off-automatic)
This is a combination of both. With a SPDT center-off switch, you can have both worlds. With the switch in position one, the power flows like diagram #1. With the switch in position two, the power flows like diagram #2. With the switch in the middle, the lights are off.
