Murf300,
I have not posted a reply in while. Things have been very busy around here, but I can completely identify with your problem. I've encountered this several times over the last seven years. Plastic windows on the BMW Z3, Porsche Boxster, Mazda Miata, Jeeps, etc. I have rarely seen them in great shape. I use the Acrylic-Werks Polish to restore these windows and have never let myself or the customer down. Steps:
1) Take wet 100% cotton washcloth and ring it out completely by hand. Fold in half, then fold again (small square).
2) Apply about a half dollar size of polish onto the cloth.
3) Start with exterior and wipe polish on in a criss-cross motion both N & S and E & W. Use the side of your hand as opposed to finger tips.
4) Watch your cloth for gray or gold residue. Work that plastic until your cloth is comming up clean. Leave the polish on, do not buff.
5) If you have a clay bar (I use PRO Clay Bar-Gray and the PRO Wipe Out as a lubricant), if not you can check around for a Clay Magic Kit, here in San Antonio the price has dropped to $9.95.
Spritz clay lubricant onto the polish that was left on the exterior window.
6) Now take the clay bar and wipe it over the surface with mild pressure in the same motion that you applied the polish in.
7) When the lubricant has dried, standing on the side of the car, take the clay bar and again with mild pressure set the clay bar in the middle and pull towards yourself. As you pull through try to remove everything from the surface, leaving a clean surface behind.
8) Now, go back and follow steps 1-4. At this time continue looking at your cloth to make sure you are not getting anymore gray or gold color in your rag. If the washcloth is clean proceed.
9) Grab a clean wet washcloth and ring it out about half way. Wipe the surface leaving it damp with water.
10) Take either a Dry 100% Cotton Diaper or MF Cloth and buff surface until dry.
11) Do the same process on the inside.
Now you may think this is a lot of work, but it isn't. Exterior will take about 15-20 minutes and the inside about 20-25 minutes because of the angles. Before you think that is too much work, just imagine how hard you have to work too replace that top at $1,000+, and what would you do to the new top to prevent it from doing what the old one did?. The nice thing about the effort put forth is, the window will not fade between applications, so future maintenence will be a snap, about 15 minutes for both sides.
I hope this helps you, because it sure has made my customers extremely happy.
Mark G.