Vinegar should be #1 against hard water.
You can try a razor blade too. Like taking off an old sticker. Be super careful not to catch an edge, it will cut into the glass if you do it right. I will leave scrape marks if your not careful. Like anything, make a test spot somewhere you won't be looking at it every time you drive your car.
You can look into CLR too. I wouldn't risk it on a car but it works great in bathrooms.
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