Rear view mirror
In this simple element can be integrated a lot of technology required for other functions to work correctly.
Mirror detects condition in which driver can be blinded by a following car. Check more details for Auto dimming rearview mirror.
Manual lever is used to dip the mirrors assembly so the other mirror takes the position of the first one.
Ambient light sensor
Remote garage and gate opener.
Some car makers (like Volvo) integrate compass into rearview mirrors.