Re: Chroming Plastic
In fact you can chrome plate plastic, so long as you use the right plastic. Chroming is an electro plating process, where a current is passed through the part while it is suspended in a solution of chemically dissolved chrome. The chrome in the solution is depsosited onto the part. There is no particular heat involved. But as you can imagine, for this to work the part must conduct electricity, which plastics in general are bad at. Special plastics have been developed which do conduct electricity, and so they can be chrome plated. This is a fairly common process today, along with vacuum metallising.
Unfortunately, neither process is likely to be successful in rejuvinating your grill. Best bet would be to find a replacement. Or if you're happy to experiment, you could try the 'chrome effect' aerosol paints you can buy from hardware/paint/automotive supply stores. It might even work!