I know exactely what lucerne06 is talking about. And too, I can't think of any product that only works for awhile. What the grey panels on lucerne06 PONTIAC Vibe needs is to be repainted and there are some pretty good DIY products on the market that will actually do a better job that the "Shine Products" that others are mentioning here.
The area of the greay panels need to be stripped of wax/grease/oil and shine products. Preped, primed and repainted. It's not a difficult job but, like painting anything, you must first mask off the rest of the vehicle.
I recently did a Caravan front bumper where the bumper cover was starting to fade and also had wind waves in it from high speeds. I cleaned it, used a clear primer/activator and reshot the bumper only giving the bumper a fine dusting to cover well. No need to apply heavy coats.
Of course in a couple of years to come, it will need another coating. Looked great! Whole job only took about an hour.