Rear Window Fogged (unrestored '65 Signet Convertible)
You have a plastic rear window, my guess is it will never be clear again.
You could try the headlight cleaner stuff
But I doubt it will work.
As for the zipper, zip away, as long you are comfortable with it never going back up.
I don't mean to be the voice of doom and gloom but there comes a time in every convertibles life to get a new top!
Also if the top is in good shape the curtain with the plastic window can be replaced.