Does your vehicle have an Electrochromic Rear-View mirror (ECM)? It automatically dims when it senses light from behind the car, reducing glare that can create vision problems for those driving at night. Look for a dark black area near your rear view mirror, or wires behind it
Does your rearview mirror on the windshield dim automatically when a car is behind you with its head lights on?
Does your rearview mirror automatically dim when at night when headlights are on behind you?
The answers to the above questions will help you determine if you have this feature or not.
Below are some vehicle examples of vehicles we know have this option: