Drove in to work today and it's about 10'C, and the mirrors were definitely warm, so I think they probably come on with ignition - if they're thermostatically controlled, then by rights, there is no major reason they can't be fed power all the time, and trip in when it gets cold.
That said, it was raining when I drove in - so maybe they ARE linked in to the use of the wipers OR temperatures below a certain level.