Door Sill Plate Door sill plate on a car is found where the door meets the frame. Door sill plate makes the cabin more exciting and also protecting the frame. The sill plate can take on many forms from a sheet of aluminum to combination plastic and aluminum plate. Door sill plate is essentially in place to protect the frame of the car from wear out, provide traction and of course as a style element. Protection and style all together with door sill plate, from Otoproject.