Hey guys, wired up my Glowshift gauges the other day (gott'em into the A/C control area

) and they're just too bright. Decided to go ahead and hook up the dimmer functionality.
Just need to know a good place to tap in for a switched 12v source that'll toggle when I turn on/off my headlights.
I've searched for 15minutes or so and couldn't find an answer outside of "tap into the headlamp 12v switch" -- not descriptive enough for me.
Thanks in advance, sorry I'm sort of a newb