OK. The gray/silver extruded piece is the threshold. You will probably need to take it off anyway to get at the area under the white plastic. Also the rust stain may be coming from your door. Take a look under there.

Regardless, once you've found the rust, use the POR-15 to seal it.

__________________
Steve

1992 300CE Sportline Sophie
1990 300E Emma - in the family
1979 240D Josephine - sold, but not forgotten
2004 Pacifica AWD

http://web.mac.com/dakota/Mercedes/Home.html