The washer jet heating comes on automatically with the rear window and mirrors I presume. Now, the problem when it's cold and you don't put wiper fluid but clean water in the tank, is that it freezes, in the pipes and if it's cold enough in the tank too, so then heating the jets doesn't really help a lot. You guys should be careful with this, because when it freezes and expands, it might destroy the little tubes the wiper fluid travels in! So, use wiper fluid (with alcohol in it, blue stuff, don't drink it!) in the winter if there's a danger of less than zero degrees! In the uk you could mix water and wiper fluid 50/50 I think, because it won't ever be -15 anyway, right?