After replacing my rear deck speakers, I'm trying to re-install the rear seat in a '01 and I'm ending up with a 3/4" gap between the rear deck and the back of the seat.
I have been looking at the service manual and it hasn't been much help and I'm scratching my head on what to try next.

 

There are hooks on the seat back that need to slide down into place; if not there will be a gap there and the seat back will be loose. You need to hold the seat back up a few inches and make sure it's flush with the frame in back and then slide it down. And make sure hooks go in respectful holes.