I've used this method on motorcycle visors with sucess, but mainly to stop them steaming up rather than good vision.

I am perpetually bugged by dirty car windows, and I usually do it one of two ways:

- a local proprietary screen cleaner foam manufactured by Mer in Frankfurt, which I doubt you'd be able to get. However, I'll let you have a tin if you come over to the owner's meet in July
- bucket of warm water, about 1/2 litre of white vinegar, a piece of towel and a moist chamois. Wash the windows liberally with the mix, then squeegee it off with the chamois. Wring it out each time you wipe off. Takes practice not to smear; doesn't get rid of the arm ache.