this an addition to piercedtiger's bed light and underbody light mod.
so my idea when i started this mod was i wanted the option for everything to light up when i unlock my truck or open the door to get out.
it took a little bit of disassembly to investigate how to do it but what i found was going to the dome light in one constant hot and two grounds on is always grounded and the other is on a relay so the light comes on under certain circumstances. this is the wire i tapped in to we'll come back to it.
so the first thing i did when i pulled my lights out of the box was chop off the plugs as i did not want to use the control box that came with it. wired all the pos+ wires together this will take a little bit of trial and error with a battery to find which wires are which. there is only one neg- wire in my set of lights. I did not use relays for this mod as the lights only draw 3 amps total thats as much as a normal relay itself wasn't worth the extra wiring.
so i wired the pos+ side of the lights directly to the pos+ power point in the fuse box under the hood the neg- side of the lights go to the center point on my double throw switches.i then wired a constant ground to one side of the switch. all the wires came in the cab through a ruber gromit in the floor on the drivers side.

this is where that dome light ground comes in to play i found the easiest way to access it was in the drivers side piller its a light blue wire for my truck it may be different depending on model and year. this ground was wired to the other side of the switch.

this way you have the option of having the lights on weather the truck is on or not. (up position) off (middle position) or the lights with come on automatically with the dome light.(down position)
and here is a basic wiring diagram to make things a little more clear

and a video of it in action