The trim is indeed stainless steel. Polish it like you would polish any stainless steel, just be careful not to ruin the paint along the stainless.

That 4" long driver side cowl trim piece is missing about 90% of the time, finding one in good condition will be difficult and likely, expensive. The passenger side trim piece is different. The part fits all Plymouth body styles between 46 & 48 that came with that trim level.

It is required (well, OK requested) that you post pictures of your car.
Gene