Maybe I am wrong but I did not put in an "interior plinth". First, my mirrors were further back behind this open area where everything was inside the door. Second, I used rivnuts with washers on the back of them and inside the door skin. This provided strength for the rivnuts, to mount the mirrors. I could access the area behind the mirrors inside the door by rolling up the window and removing the door card. Admittedly there were no instructions in my Tex mirrors but I did not take it that any part of the kit was for interior plinth.
Whoever drilled that original big hole made a bol