Rain-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.
There is an LED and a photo diode inside the rain/light sensor. The light from the LED is reflected off the detection surface of the windshield and then is received by the photo diode. If waterdrops are present on the outside surface of the windshield, part of the light from the LED goes out through a waterdrop without being reflected off the detection surface, so the photo diode receives a reduced amount of light. The rain-sensing wiper system determines the intensity of rainfall based on the rate of the reduction in the amount of the received light.