Well, it's normal for the Nav screen to dim when the headlights are on, but not to the point of barely being able to see it. When you say "brightness knob", are you talking about the one that adjusts the brightness of ALL the dash lights? That's the wrong one.
You can adjust the Nav screen brightness via the SETUP menu. In addition to a touch-screen brightness scale, there are preset buttons: Auto - brigher w/o headlights, dimmer with; Day - always brighter (always the same); Night - always dimmer (always the same).
You can adjust the Nav screen brightness via the SETUP menu. In addition to a touch-screen brightness scale, there are preset buttons: Auto - brigher w/o headlights, dimmer with; Day - always brighter (always the same); Night - always dimmer (always the same).