Which Wire For Accessory Light With Dimmer?

This is my first post here, so forgive me if this question has been asked and answered. I looked around first, but I couldn't find a thread pertaining to this.

I need to locate a wire behind or under the dash of my 2004 2500HD that will power an accessory light and will dim when the rest of my cluster dims. I tested about seven wires from behind the dimmer switch, but had no luck. I would say there are probably another twenty wires that I haven't tested coming from the dimmer. I didn't want to cut every wire to find out that none of them worked, so that is how I found my way here, to this forum.

Please tell me there is one wire that is easy to get to.

Thanks,

Bruce

do you have a wiring diagram?

I don't have a wiring diagram. Do you know of any online?