Im running the stock steelies and stainless chrome trim rings with napa/balkamp dust caps.
This combination looked great for awhile, but
over a period of about 6 months, the black steel wheels started to fade to almost a
a grayish color. Ordinary washing doesn't help. I suspect when I ran the car through the automatic car wash once, some solvents might have reacted with the wheel paint, or perhaps UV exposure (but I live in boston, and the car is less than a year old), so Im not really sure how this happened. They just kinda degraded..

Does anybody have experience on how to make the black rims look black
again short of re-painting? Maybe a polish, rubbing compound, or black wax?

See info. here...

http://www.scionlife.com/forums/viewtopic.php?t=178276

I used scratch X first then 2 coats of Nano wax. Brought the black back out. Maybe just wipe em down with rubbing alcohol and shoot them with some clear wheel paint for longevity.