Okay, for starters I bought my Jeep YJ with windshield mounted KC's installed, but not working. Approximately 5min of troubleshooting lead me to realize the wires were chaffed through and shorted out. This lead me to rewiring the whole mess. As I was looking at the PO's wiring job I became very disgusted to find that he had pulled the power for the lights from the clock circuit by jamming the stripped wire into the fuse end: not good for offroad lights.
Thats when I started looking into wiring schematics and planning out my method of doing it right. To start with you need to know the power output of your lights. Mine were 2 @ 100W but I calculated at 130W cause I might upgrade at some point. Next you need to decide how you want to control your lights. I wanted the ability to shut off my offroad lights along with my brights for the times I am out in the sticks without other drivers to worry about. This requires splicing into the dimmer wiring. I also wanted to turn my lights on independently of the headlights for whatever the reason may be. Below is my schematic I ended up making to guide me.
260W of power gave me a single 10AWG power wire feeding the 30A relay I bought at RadioShack. I posted a picture of the type of relay I used at the bottom.
The rest of my wiring was 14AWG mainly because I think its best to go a little heavy on the wire and fuse it appropriately, and its what I had on hand already.
These wire sizes are from my own calculations given the facts taht I am powering two lights from one relay with no single wire being longer than 6ft. Wire length is important to think about when planning electrical installations, just like current and voltage are important. I used a 25A fuse at the battery to limit the max amps my lights could pull, and it is lower than the rating of the wire and the relay for good wiring practices. The "hot" side of my in-cab switch has a 3 amp fuse at the battery, which is well below the power rating for the switch I bought and the wiring I used. Remember that the switch only powers the relay, which requires very little current.
I chose a 3-way toggle switch with a center "off" to turn my relay on. One side of the "on" comes directly from the battery, and the other comes from the high-beams hot wire (its only hot when the high beams are on). I used a multimeter to determine which wire to splice into right by the dimmer switch on the steering wheel.
Be careful about an installation such as this because if you accidentally leave the offroad lights on with the Jeep off, you'll drain your battery quickly.