On a low OBC (those without messages), there's a light sensor that only control the odometer area (odometer, outside temp, ave. fuel consumption, etc) without affecting the rest of the instrument cluster. The backlight for that display changes as you drive through an underpass. The rest of the instrument cluster brightness is control by the rheostat. I've noticed that the radio MID also has its own light sensor. I'm just saying that a bad light sensor (can no longer detect light) could just make the system think that it's always night time, thus dimming the display to 25%

Does the backlight uses standard bulbs or LEDs?