Door Panels

Alright, so this is probably an easy question for those of you who have been through this. However, I am completely lost! On my 72 Chevelle there is the door panels that go on the doors, and those are relatively simple to put on. But, my question is how do you do the rear part? It looks like the seat covers up most of the metal on the side and then a plastic piece is supposed to cover up the rest. This plastic part has no ability to connect to the car though! Am I just being dumb or am I missing something here? The piece has an armrest and ash tray in it so I am pretty sure that I have the right piece. Please help!

Mac