Martin, that wasn't a dig at you...it was one at Squeaky!!
If you could capture rain water, particularly rain water that has been falling for 30 minutes or so, and you could also collect in a manner that would not get it contaminated then you could most certainly use it to clean windows with.
Plastic trim does dull with weathering, but the back bumper area will soon look ropey, hoses being dragged over, dirt gathering, feet stepping on it when you clamber into the back and so on.
If you want to prove that water at the purity level used by window cleaners can take the glaze off either plastic trim or paintwork, show me the SCIENTIFIC proof, I'm not interested in someone saying that they heard someone talking about something they read about someone who said....well, you get the picture.
I've read many times of people claiming that pure water is more aggressive than ordinary tap water (or rain water) really??
Give me a link to the neutral research papers and their conclusions and I'll eat my words.
We only measure the water purity in parts per million, truly pure water is measured in parts per BILLION!
A vehicle washed in tap water from a very hard water area...now that will cause more problems than a window cleaners pure water!
Ian