For whatever reason, my navi screen no longer dims at night. I believe the dimming is connected to turning on the headlights, but that has no effect now. The dimming feature is set to auto. The only way to make it dim is to manually turn it to night. Has anyone else had this happen?

 

have you pressed the max button? if so it will keep the setting and not dim even when you turn on the headlights. next time you get in the car try pressing the max button after you turn on the headlights.

 

The "MAX" button acts as a toggle as to the brightness of the gauges and the day/night display on the Navi. You didn't specify whether your gauges have been dimming properly, but I suspect that they are not. Try using the "MAX" button after turning on the lights as caball88 describes, and then if that doesn't do anything it sounds like you would have a problem.