how to really clean rear window

Apropos of the recent thread on rain-X removal and Glass Wax...

I have a Porsche 944 which as you may know has a large, almost
horizontal rear window. The problem that I have is at night it's near
impossible to see through the rear window with someone following me due
to glare. The lower third of the window either has some deposits on it
or else has become mildly etched due to sitting out in the rain over the
years. I've done the Bon-Ami thing and actually just the other day the
window was sparkling - but it frosted over last night and when the
frost melted all the water that ran off revealed all the old marks on
the window. I guess it's kind of hard to describe without posting a
pic, but imagine an old shower door that's hard to clean. Is there any
help for it besides having it polished? I'm hesitant to try to do it
myself as that piece of glass literally costs almost as much to replace
as the car is worth.

thanks,

nate

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