Hi Paul, the only power to the switch is from fuse #6 10amp is for the backlighting, the switch provides an earth to the windscreen heat relay (mounted on UK passenger side front suspension strut mount) the power for the heating element is from a 60amp inline fuse by the battery, the relay will only engage if the engine is running and the alternator warning light is "off"
If you would like a copy of the wiring diagram and component locations contact me on nige110162(at)hotmail.co.uk
nigel
September 2010