There should be a sill plate extension between the sill plate and the rear seat. In the pic below, I am not sure if it is installed correctly or not. The plates are LH and RH specific but are an L shape, so can be installed with the long or short part of the L down depending which side of the car it is.

The extensions are reproduced if you do not have them.

If anyone knows which way is correct/original, please chime in. I have tried them both ways on my car and neither one looks right to me.

I see in the pic above it is installed the same as my pic, so perhaps that is correct.