[bnorman]

Do not use Windex or any other product for glass cleaning. You need to use a plastic polish like Plexus. The cleaners in windex and other glass product remove the polymers in the plastic and will make you window brittle discolored. See below from Car Care Specialties:

Plastic Rear Window Care:
The last item on your agenda is the clear (or maybe not so clear) plastic rear window. Do not use glass cleaners on a plastic window, as they are too harsh and may actually damage plastic. There are several cleaners and polishes on the market that are specifically designed for plastic windows. Of the many that I have tried, the Mequiars, Renovo and One Grand products seem to work the best. I have found that Meguiars #18 Clear Plastic Detailer will help maintain a plastic window that does not have any scratches or clouding in top condition. Make sure that the window is clean and free of dust. Spray a small amount of the Meguiars #18 on a soft, 100% cotton flannel cloth and work into the surface, using linear strokes, left to right, right to left. Dont go around in circles, as the resulting swirls are very visible. Immediately buff off with another soft, 100% cotton flannel cloth. Do not allow to self dry. If there are scratches or discoloration, try either the two step Meguiars #17 Clear Plastic Cleaner followed with Meguiars #10 Clear Plastic Polish or the one step Renovo Plastic Window Polish or One Grand Plastic Cleaner & Polish. Apply a very small amount of either of these to a soft, 100% cotton flannel cloth. Work into the plastic with a linear motion, left to right, right to left, until all that remains is a faint haze. Immediately buff off with another soft. 100% cotton flannel cloth to crystal clarity. Do not allow to self dry. Depending upon the condition of the plastic, you may have to repeat several times. This will usually remove most light scratches. If not, you may have to resort to a more aggressive cleaning product. I have found that 3M Imperial Hand Glaze will remove almost all deeper scratches. Use a small amount of 3M Imperial Hand Glaze on a soft, 100% cotton flannel cloth and work into the surface with linear motions, left to right, right to left. Immediately buff off with another soft, 100% cotton flannel cloth. If the discoloration or scratches resists the 3M treatment, try adding a small amount (about the size of your pinkie nail) of P21S Multi Surface Finish Restorer Polish to a 100% cotton flannel cloth saturated with 3M Imperial Hand Glaze. Mix together on the cloth and gently rub the window in a linear motion and buff off with another 100% cotton flannel cloth. Follow with an application of Meguiars #17, then Meguiars #10 and you are done. If none of the above work, it may be time to call "1-800-new-window".

One of the problems many convertible owners encounter is a crease in the plastic window from repeated folding. The easiest way to minimize this problem is to use a super soft, 100% cotton blanket or towel that is a little larger than the rear window. Place the blanket/towel over the window as your fold the top down, so that the blanket/towel is folded within the rear window. The thickness of the blanket prevents the plastic window from creasing and also keeps the two halves from rubbing against each other. If you have a crease in the window, try parking in a very sunny area with the sun shining directly on the window. The heat will usually soften the window enough that the crease will disappear.