After looking at your picture again it looks like you have a custom harness. It might make my post even more confusing. Here is a wiring diagram
It's pretty simple. Just make sure yours looks the same (proper wires going to the correct prongs and sources). It's going to be harder running wires cleanly and having the lengths of wire you need to put stuff where you want it. I have a fuse block in the cab which makes it easy to wire up accessories. It's 12V hot and fused. I've got my relay about 6" from the box so all my wires are short and organized. The longest wires are from the lights to the box and from the switched 12v to the box.
It might be worth it to go to Radio Shack or the like and pick up some split wire loom and small zip ties. It'll make the install nice and clean and protect your wiring. You don't want something chafing and shorting out. It's worth the couple bucks.