Mud & sand in my 2001 Sienna doors

My 2001 Toyota Sienna seems to have a design flaw in the bottom of the
doors, which allows mud, sand and water to splash up under and almost
into the passenger compartment. This model seems unique in that it has
a plastic piece along the bottom of the front and cargo doors, but no
rubber gasket. Come winter, this problem will allow salt and more mud
in to further rust nuts and hinge mechanisms. I'm wondering if anyone
else has had this problem and how to fix it. Two Toyota parts guys have
acknowledged something appears wrong, but neither had any ideas, or
parts, to correct it. The sales people, of course, say they've never
had a complaint.