I had the same problem with my headlights, the truth is no matter how much you try to clean them they still wont look new...I suggest buying new ones on ebay, cheap!!! i bought mine brand new...50 bucks it makes your car look brand new as well...haha

 

We have 2 Caravans and as I mentioned I used the Walmart kit. On the other one, we used 500 grit, then 1000 grit, then a paint polish--they looked like brand new, but within a week here in AZ they started to turn hazy. It looks like we need a UV resistant coating which the Walmart kit has.

 

If the problem is on the surface, you can wet sand with 400 grit (or coarser depending on what you need) wet or dry silicone carbide paper (the black stuff). Follow this up with 600 grit. Then use rubbing compound, then use any good combination polish/wax. The original sanding will leave the surface foggy, but restoring the clear surface is merely a elbow grease process involving much polishing with finer and finer materials. But each time you have to work long enough to remove the scratches from the last effort.