Back Seat Problem
The strap doesn't release the seatback to the upright position. I strongly suggest you don't try to fix it from the back. The best way to fix it is through the cushion on the front.
First, open the back via the zipper and pull the cover to open as much as possible. You'll see the strap coming down under a plate. There is a lever under the plate that connects to two cables. Trying to fix it from this side is nearly impossible. You need to get the seat back upright first. Either with your fingers or with a clamp, secure the two cables against the plate and slowly pull the strap until you can get the seatback to release. Once released, bring the seatback up to the normal position and lock it upright.
Now go around to the front side of the seat, pull the seat cover up and look at it from the front side. There are three screws holding the plate, unscrew them. You'll see the two cables with the broken plastic ends, ours were yellow. The ends will have likely slid down the cables. Pull the ends up and push them back into position on the metal brackets, but one notch higher. See the attached pictures. We put a dab of construction adhesive on them to keep them from working loose.
Screw the plate back on and you're back in business.