I've installed a guage behind my dash glass and need help finding a dimmer wire behind the dash to tap the gauge into.

The gauge has a purple wire and a red wire that I am confused as to what I should wire these to behind the dash.

These are what the instructions say:
Red wire: Ignition wire, hookup to a switched +12 volt ignition source.
Purple wire: Dimmer wire, hookup to a switched light source.

The gauge lights up when only the red wire and ground wire are connected, but I dont understand how the purple wire makes it dim. I'm assuming when no power is going to the purple wire the gauge is bright like during the daytime when the headlights are off, but when there's a signal to the purple wire the gauge dims like the dash lights when you turn your headlights on.

I'm assuming there are wires going to the clock and gas/temp guage that I can tap this gauge into to get it to dim, but I don't know which ones are which. Any help would be appreciated.

 

it's the fat (well fatter than the other wires) red wire with blue tracer

 

awesome, thank you.