I have found that the Auto Dimming Rear View Mirror does not dim because the light sensor on the mirror is too high.  At night the sensor is in the shadow of the overhead console.
The Folks at GM-Chevy Volt Forum have found a cure for this situation!
Auto-dimming-mirror-not-auto-dimming
It is simple, just rotate the Auto Dimming mirror 180 Degrees so that the Light sensor is on the bottom of the mirror.

Now, the sensor is no longer in the shadow of the overhead console when someone is behind you with their headlights in your eyes.