My dealer said you had to activate the dashboard demister switch AND have the mirror switch on the demist setting for the mirrors to clear i.e. you could demist the rear screen and mirrors, or just the rear screen - you could not demist the mirrors alone.
I can't say I've had occasion to demist the rear screen or mirrors yet, so don't know if what I was told is correct. The mirror switch has been set to demist since I got the car from the dealer (four months ago).