Inside rear deck of duster

I agree, sounds like you have a fold down seat car. Nothing goes there, the seat back is held in place by latches on the wheel tubs. You can see that set up in the pictures above.

If your package tray looks like this one, you have a fold down seat car. Nevermind the notes, I borrowed this picture from the thread on converting my standard car to a fold down seat.



My thread is here. Probably not useful as a conversion thread for you, but there's a lot of pictures of both standard and fold down components, if you don't have everything already.