You can't really brighten it up. They are aluminum but are clear anodized over a "bright dip" type of coating. The only way to get it bright and shiny again is to strip the anodizing and bright dip off and then polish the bare aluminum. Problem with that is it then needs constant care and polishing or it starts to turn dull and whitish fairly quick. I stripped and buffed all the window trim on my Aspen, and I need to use a polish (I use Griot's Garage Metal Polish) every couple months, and it spends 95% of it's life indoors. I tried an aluminum protectant (like Zoop Seal) but it didn't really seem to help much.
I know, way more information than you wanted to know.
I really thought the chrome would polish up fine but i guess i was wrong. I got the windshield trim (still haven't put it on) from it and polished it with some Never-Dull but it didn't really do much. The strange thing is the bumpers and the grill are shiny, so go figure