As Winu said. The mirror heaters use PTC elements. This means that the resistance increases as the temperature rises, maintaining a constant 40°C give or take . They are fed by an ignition live so are permanently on.
When the same system was used by early Rover vehicles, there was a thermostat in each mirror shell, preventing the elements from heating unless the air temperature dropped below about 5°C. This thermostat led to unreliability of the heaters, so was dropped from all 95 onwards models. From then on the heaters were fed with ignition live, so were permanently on.