Need a plastic part for a 1999 Mazda Protege Need help ?

My mother in law owns a very nice but aged 1999 Mazda Protege LX 4 door sedan. She tries very hard to keep this vehicle spotless. The right rear door frame has an aluminum plate on the entrance of the floor rocker panel and in part of the body there is a plastic snap in insert. I have no idea what it's called, but we need a replacement in color brown. I guess it's a trim piece. Can anyone help ? Thanks in advance.