On my Pug, when it's switched on, it has a daylight sensor facing the front windscreen, ensuring it will not "dim" by sunlight, and a rear facing photo-cell. the amount of light falling on the photo-cell will depict how much the electro-chromatic glass will dim.
had it for last 18 months. Brill piece of kit, also got rain sensing wipers, so will defo be ordering the package as an option for the new one