My car's harness (02 CLK55, comand 2.0) doesn't supply illumination so the common practice is to tap into the wire that illuminates the cigarette lighter - it's +12V when the lights are on. It was working, now it's not. I only changed it once, so I don't recall exactly, but I'm certain that on the picture adjustment > dimmer page the icon changed from sun to something else (moon?) when the headlights were on, switching between day/night settings based on the voltage on the illumination wire. I pulled the HU and checked the voltage - it's still +12V when the lights are on, 0V when off.