It might be something to do with the SAT-NAV light setting. The standard setting for the system is for the SAT-NAV back-lighting to dim when you have the car lights on- I personally do not like this as the screen goes to dark so I have overridden the ?Automatic? setting to the ?Manual? setting.
To do this I went into the SAT-NAV system setting menu and there is an option for lighting or brightness which allows you to select either auto or manual.