Light Sensing Principle
WARNING: This page is about a different car, the 2010 Suzuki Kizashi. However, it is still accessible from the selected car via links, so may be relevant.
The rain/light sensor incorporates 2 light sensing devices, one for sensing the light from the front and the other for sensing the light from above. This two-sensing-device configuration enables the sensor to determine whether the detected light intensity change is temporary or not.