You might wish to try a trim restorer product like one of the three listed below.
I swear by Poorboy's World Trim Restorer.
1. Mothers Back 2 Black (Walmart type stores)
2. Meguairs Gold Class Trim Restorer (autogeek.net / other car detailing websites and possibly Walmart type stores)
3. Poorboy's World Trim Restorer (autogeek.net / other car detailing websites)
1st two products I had smearing issues after it rained, could have been how I used it, not 100% sure. On the flip side I've had great luck with Poorboy's were other folks have had issues with it, so go figure.
Only way to know is to try it yourself.
Both sites listed in my signature line have EXCELLENT forums for this kind of issue. Check them out (they are the equivalent to this site for car detailing).
Mark