I just got through with this whole nightmare. I have a 1994 325 convertible, and live
in the desert, need I say more. Anyway here are your options. If the plastic is not
to bad ( cracked or yellowed ) you can take it to a detail shop and have them use a mild
polishing compound with a buffer on the window. I even tried this myself and it works.
There is a downside however. When you buff the plastic it takes off the top layer, thus
clarifying the view. The bad part is that it is more susceptible to the sun, and will yellow
easier. You can slow this down by applying a good clear wax with UV protection in it.
This will work for awhile. Eventually you will have to replace the it with a new plastic
window. These can be found easily on the internet or Ebay, (I even have an extra one)
One last thing. When you get a new window, pick up some Meguirars Mirror Glaze
clear plastic detailer. It works wonders on plastic windows.
Hope this helps,
Jay