Two mirrors will auto-dim on the H2, the rear view mirror (in the center of the windshield area) and the left (driver's) side mirror. They are connected into the same sensor which is located in the bottom right of the rear view mirror.
You should also see, when the ignition is on, a small green light at the bottom left of the rear view mirror. When that light is on you are in the "electrochromic" (auto-dimming) state.
You can turn the electrochromc mode on or off by pressing and holding the mirron TEMP button for six seconds when the ignition is on.
I think the auto-dimming feature is pretty cool. I had it, on the same mirrors, with my Jeep Grand Cheerokee. I have the feature on my BMW as well, but those Germans didn't see fit to include the driver's side mirror in to the feature. So when a clown with his brights on is passing me on my side I get blinded.
(Yeah, make your jokes now, about why is a clown passing a Beamer.
)