The build sheet is nice to have because it has the date of manufacture, the plant where built, the name of the dealer the car went to.
To get that seat out, put the front seats forward, and crawl in the back facing the rear. Push back on the bottom of the seat cushion. It will pop up. You will see how it is attached once it is out.
On all the cars I have found build sheets in, they have been in the springs of the front passenger seat.