I'm pretty sure the brightness is controlled via GM LAN/CANBUS, as there's a value for brightness in the Test Mode screen. If it was just a binary input (on or off via a wire) you'd be laughing.
You could do something clunky with some relays, a normally closed relay with 12v from the headlights to the coil, switching between a 12v ignition live and a lower voltage.
Bookmarks