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.