Whenever wiring off road lights, it's important to add a relay to the wiring. Failure to do so would cause excessive heat in the wiring and switch and create a risk of fire. This is caused by the amount of power being drawn by the high output lights or accessories. Anytime you want to switch a device which draws more current than is provided by an output of a switch or component you'll need to use a relay. In some cases you may need to switch multiple things at the same time using one output. A single output connected to multiple relays will allow you to easily accomplish this.
A relay will be one of two types:
Single Pole Double Throw Relay which consists of a coil (terminals 85 & 86), 1 common terminal (30), 1 normally closed terminal (87a), and one normally open terminal (87). When the coil of the relay is at rest (not energized), the common terminal (30) and the normally closed terminal (87a) have continuity. When the coil is energized, the common terminal (30) and the normally open terminal (87) have continuity.
Single Pole Single Throw Relay which consists of a coil (terminals 85 & 86), 1 common terminal (30), and one normally open terminal (87). It does not have a normally closed terminal like the SPDT relay, but may be used in place of SPDT.
The following is a nice pic I found of a common wiring setup using a relay for a pair of lights:
The relay switches the heavy current to the fog lights with big wire, while the dashboard switch wiring can be very small. This keeps high current and large wires out from under the dash and in the engine compartment where they belong. A typical set of 55 watt lights will draw about 8 amps for the pair. The dash switch will pull only about 0.3 amps to operate the relay. Be sure to run extra ground wires from the fog lights themselves. Don't depend on the cheap spring or case ground to work. Take the light apart if necessary, and add a real ground wire. Run these to the frame somewhere handy. This will keep the lights from being intermittent.
The only other thing you need to be concerned about is using the right gauge wire for the job. As noted above, the switch is only going to pull about .3 amps, so a light wire, 18 or 20 gauge is sufficient, but the rest depends on the amp draw of the lights.
The first thing you have to do is determine the current you have to carry. For DC circuits, that's relatively easy. Auxiliary fans, fuel pumps and things like that are rated in current draw - Amps. But most lights are Watts. The power requirement in Watts will be printed right on the bulb or stamped in the base. To come up with amps use the the following formula.
Current in Amps = Watts/Volts
This will tell you both what fuse you'll need and what size wire you'll need.
For a pair of 55 watt lights, which draw about 4.6 amps each, a 15a fuse would be plenty, for a pair of 100 watt lights (at about 8.3 amps each) you'd need a 20a fuse and for a pair of 130 watt lights (at about 10.8 amps each) a 25a or 30a fuse should be used.
For a pair of 55w lights, I would use 16 gauge wire, for a pair of 100 watt lights, I would use 14 gauge wire and for a pair of 130 watt lights, I would use 12 gauge wire. This is assuming no runs over about 6 feet, for a run longer than that up to about 10', I'd go one gauge larger.
For a set of four lights, if they are 55 watts, it's still ok to use a single 40 amp relay, but I'd run all but the switch in 14 gauge. Any set of four lights larger than 55 watts and I'd use two relays, wired to a single switch.
Another "trick" you can do is pick up a wireless relay (about $15 online) so you don't have to run a switch into the cab. If you have a "HomeLink" in your vehicle, you can simply program the homelink buttons to turn your lights off and on. I did this in my Jeep Grand Cherokee as I was only using one of the buttons for my garage door opener. Most wireless relays are 15A, which is fine to power a set of 55 watt lights. For more, simply wire the wireless relay to a standard 40A...
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