Having driven a 356 Cab for 25 years, I have one critical tip on the care and feeding of the plastic rear window;
BEFORE LOWERING THE TOP, wipe the outside of the window with a moist towel (paper is O.K.) and PAT dry. Don't wipe it to dry (although wiping may be faster, I find that it can scratch).
Recently I replaced the 24 year old Cab top. Although the fabric was faded and torn, the rear window was in good enough shape to see through (Note that mine is a late 356 Cabriolet and thus has a zip-down rear window-a little different than the Boxster). I never used any type of plastic cleaner on it.
It appears that the dust (it's there even if hard to see-just look on the paper towel after you wet wipe) acts like sandpaper on the window, and really does a job on it.
Good luck. Tom
