Revisiting this post - but I have a similar problem. The outside of my driver's side window has a weird film on it that Windex or Invisible Glass won't seem to get off. Anyone else have this? I'm thinking since I've only had my cabrio since August this is the only window I've put up and down (bank and donut drive up etc...). Maybe there's something in the window slide area?
I'm going to try to use magic eraser tomorrow morning - can't hurt?