Just to clarify:
This sensor will enable your dash lights and Entune screen to dim at night without you having to fiddle with the light control thumbwheel (located left of the steering wheel).

It does not dim or turn on your headlights.

The way I use it is to set the thumbwheel to max brightness without clicking it to the highest override setting and leave it there all the time. Check out how this functions at night (or in a dark garage) by turning the ignition ON but do not start it. Turn ON the head lights or parking lights. Your dash lights are now dim in “night mode”. Take a flashlight and shine it closely on the sensor. In two to three seconds the dash lights brighten to “day mode”. Take the flashlight away and they dim again to”night mode”. Yay.

Now to set up your Entune screen hit the Home button, hit the gear symbol in the upper right corner, (should now be in Setup menu), hit Display. As long as your headlights are still ON you’ll see the “Adjust Contrast/Brightness” menu with a “Day Mode” and checkbox. Deselect the checkbox.

Now hit the General button and with headlights (or parking lights) still ON adjust the brightness and contrast to how you like the Entune screen to look at night. Hit the OK button to save the settings.

Now you’re back at the Adust Contrast/Brightness menu again. Turn OFF the headlights and everything get bright. You are now in “Day Mode”. Hit the General button and adjust contrast and brightness to your favorite daylight settings. Hit OK to Save.

Back out of the menu and get to the Navigation screen (showing a map). With headlights OFF your dash and navigation screen are bright in Day Mode. Turn the headlights ON and everything dims to Night Mode and the nav screen goes to the dark background with white text.

With headlights ON shine your flashlight on the sensor and watch the dash and Nav screen go bright in Day Mode. Take away the light and it dims. Eureka!

Now you may be wondering what the “Day Mode” checkbox does. That’s for a third contrast/brightness setting for the Entune screen. If you were driving in the day and suddenly it starts raining and you want your headlights ON, the dash and Entune screen would also dim. Which you probably don’t want, you want normal Day Mode brightness. So you have this option to drill into the menu to get to this screen and check the box to force the screen to stay bright even if lights are ON. While you are driving. Right. A pretty useless feature since you can just click the thumbwheel to max position and it’ll do the same thing.
Or if it doesn’t get too dark during the storm, the light sensor module will keep you in Day Mode anyway. No driver intervention needed!