Front seat back will not stay upright on the right side

There is a latch and lock in place system that is atuated by lever. I provided you with the best diagrams I have to remove the backrest or components that have come loose or have broken. In some cases there may be a backrest position cable, but yours appears to be a rod that goes all the way across. This should be apparaent as to what controls the latch and lock system once you get it apart. The sytem is a rod that is attached to the lever and releases a lock that fits into the holes that you will see on the seat track. You will have to inspect and find out what is keeping the one side from locking into or impeeding its position.