Originally Posted by Jose_JN
It is for the autodim feature.
You have two sensors (one on the glass and the one you are showing).
The mirror compare the light received for both sensors and darkens the glass if there is any possibility to blind the driver.
As for the FLA retrofitting, are you sure that the rain sensor is required? In ETK I can find a rearview mirrors just with EC and FLA function, no rain sensor?