I have Tom's mirrors with the heaters wired in, with the factory wires spliced into a relay on the rear defroster so they aren't running all the time. The relay is mounted behind the drivers side back parcel shelf wall. The only downside you have to remember to turn on the rear defroster in order to adjust the mirrors. Not really a big deal since with these mirrors, they're a set-and-forget deal.
My car never sees snow or ice, but on a cool, misty/foggy day they will clear up the side mirrors in short order. The few times I've been caught in a humid fall rain have made it worth the effort.