Adding running boards with lights

I would like to know where I could splice in a set of running boards that have lights in them. My serivce guy is clueless as to where I can safely insert them into the dome light circuit so that they come on when Any door is opened.

you didn't say what year your truck is. if it's in the 88-98 range the domelight uses a negative switch. if you run a constant 12v wire to one side of the bulbs and run a wire to the switch side of the domelight it should work fine. the wires to the dome light run under the drivers side step plate(next to door) i believe it's an orange wire but you should check. if there are a lot of lights you might want to use a relay so you don't burn out the door switches.

I purchased a 2003 2500HD Crew cab.
I found the wiring color code as follows but there is a slight catch.
Gray = negative
Orange/Gray = opera lights
Orange = 12 volts
I connected to the opera light circuit and I get the lights to active as I wanted but, when I place the transmission in reverse, the interior lights (dome & runnings boards) light up dimly. The GMC dealer has no tech that has any experience that could shed some "light" on my problem.

sound like the negative you tied into is the ground circuit for the reverse lights if so it will cause voltage to feed back through whatever circuit the opera lights are hooked to. try temperarily grounding the lights to the frame and see what happens. or if you put in a diode this wouldn't happen. what the hell is an opera light anyways??

[QUOTE what the hell is an opera light anyways??[/QUOTE]
The "opera lights" are the type of lights that dim as they automaticaly go off.

i don't think it's a good idea to be taking power from that circuit then. theres a module that makes those lights delay and gradually dim. it may or may not be able to handle the extra current draw.