You could try Brasso or toothpaste, we used to use those on plastic models of planes, boats, cars etc that were built out of kits when I was a kid.

Platic is hard to re-chrome, but it can be done. Best bet is to turf the grille and get one with a stainless surround, those SL/E grilles were a pain for that, I remember them being buggered as used items in the 80's and when Rare Spares still had them NOS as late as the 90's they were often pitted in some places when you got them new.