If it is aluminum, it can be restore professionally. Here's a couple of links to an outfit that does it. One is a magazine article and the other is a link to the company that does the work. If you want to try it yourself perhaps the steps shown will help:
http://www.rodandcustommagazine.com/howto/86258/index.htmlhttp://www.kingoftrim.com/Kerry3, if your's is truly chrome steel and the chrome is shot, it's probably not going to come back with polishing. Once the chrome goes away it allows the nickel underneath to tarnish and the only repair is to strip and replate it. To determine if it's chrome steel, simply see if a magnet sticks to it.
If it's really stainless, you might just need to try a more aggresive polish. The great feature of stainless is it goes all the way through to the other side, so you can just keep polishing until it's shiny enough. I've had good luck with Semichrome when polishing stainless. I'm not familiar with 64's specifically, so I don't know if these grilles were ever offered in stainless.
