RE: Cleaning stains from plastic bumpers & side molding.
Soak them with a diluted version of Simple Green, let it sit for about 10 minutes, use a mitt, or a rag that you won't care about, give 'er a scrub, and rinse, Simple Green should take care of it. If not, the only solution that comes to mind would be paint thinner. Yes, paint thinner. Wipe on with a wet towel, wipe off with a dry towel. It may however dry out the plastic if there is some sun fading on the panels(the oils in the plastic cannot recover), then I would put some original armourall on a rag, and wipe it on the areas. Armourall will put some "oils" back in the plastic and give you that new look. I would not hesitate to use armourall at all.
Hope this helps.
P.S. If you have calcium stains from concrete dripping down in underground parking, CLR, or something of that nature will remove the stain, but the pitting may be visible.