gauge wiring

Hi i have a 2004 cavalier and i am mounting a voltmeter gauge to a pod on my a pillar snd was wondering if anyone knows which wire in the dimmer switch is the one you splice into to get the light on the gauge to work.Also is it connected to the positive side or the negative side ( this is what the instructions say).

Or does anyone have a sight where i can get the wiring schematics for it? Thanks