Seen this on many vehicles and is why I refuse to purchase any with the trim/bumper plastic not painted. It fades due to the sun and oxidation (sp?). I found these products, but don't know how good they are. The 2nd link has pics and looks to have done a good job.

Black Bumper & Cladding Coat

Proper Auto Detailing: How can I restore faded, oxidized plastic automotive cladding?

This link provides a little detail on how to keep these pieces clean.

Auto Trim & Molding Guide: clean & protect exterior vinyl, rubber, and plastic trim and bumpers.

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2006 Ford F150 Screw XLT 4X4
5.4L / 330CID, 4-spd Auto Trans
Chrome & Trailer Tow Pkg