The seat back bolts to the "upper plane" of the lower cushion. That is the magic of the seats. When you sink and rise in the lower cushion, the seat back moves with you rather than just "rubbing " your back as a standard seat does. You will need to feel around for the holes in the bracket on the upper plane of your lower seat cushion. Then bravely poke holes through your new upholstery for the bolts to go. Here are some pictures I hope will help. One shows the bracket peaking through that you are feeling for.
