My Saab has always lights on so the ILL wire has always +5V, thus the F910BT was constantly in night mode when set to "automatic". By pressing the MAP button 2-3 seconds, the brightness could be adjusted but I didn't like that... I wanted it to be totally automatic in function of the real day light availability.
I built a circuit that has few transistors, a relay and light controlled transistor. By putting the ILL wire through this circuit, the F910BT now switches to night mode when it's dim enough 
I used this kit:
http://www.futurekit.com/2009/index.php?option=com_content&task=view&id=210&Itemid=174
Really cheap, really easy to assemble, really easy to install and a great value! 