I believe the backlight is always "on", as it is a backlit LCD display. I believe the backlight is dimmed when the headlights are turned on for night viewing. Before I replaced my Magnadyne with a Kenwood, I pulled the unit and cut the blue (as I remember) light sense wire. I found that the display was dim to start with, and I preferred max (daytime) illumination all the time. As I remember, on the main screen was a "light bulb" icon which, when touched, would switch the display between bright (daytime) and dim (night) level of backlight illumination.
2012 Solera "S"
Calif SF Bay Area