Rearview mirror: working or not?

I purchased my 2nd Prius (2010 Prius Five/US model) a week ago and it comes with Auto-dimming rearview mirror.

It seems that dim is constantly on. It's hard to see traffic in mirror in that case.

Is ambient light sensor on that car is faulty?

Every time when I turn on my headlights than my dashboard is dimmed half-way (I checked brightness wheel and it's on 100% brightness) and navigation screen immediately goes to night theme.

I was trying to emulate light by covering sensor at the back of the mirror using my finger and putting a lot of light from iPhone's flash into front sensor: no changes in dimming occurs.

Please advice is that expected behavior or not. I have Prius v 2012 and when I enable headlights my dashboard isn't dimmed. It only gets dimmed when I cover ambient light sensor or let's say go to tunnel while driving.