on the newer Benz that have the round fasteners - two piece round plastic with a center section that elongates into a screw. The trim tool remover is a handle with a flat -Y- shaped end. I get mine from EASTWOOD they have a web site www.eastwood.com. They come in both plastic and metal.

The tool just slips behind the entire fastener and as you push it deeper into the -Y- the fastener will pop out.

Don't rip at the felt trunk liner cause the liner can tear