They work by a direct live feed from the battery to a relay which is tripped by a manual switch on the dash. The manual switch is powered off the ignition circuit. In the aluminium block there is a standard glow plug and a temperature sensor which cuts the power to the glow plug when a predetermined temp is reached.
This other type works in the same manner as the one above but is connected into the cooling system as well so the fuel is always warm and again when a predetermined temp is reached the electrical side of the heater switches off.
€70.00 and €85.00 respectively.