I can't help with your backrest issue, but I do know what that cable does. The cable is for the seat belt height adjuster. If you look at the bottom of the seat on the door-side, you'll see two metal tabs with a hole in each. They are at different angles and I'm not entirely sure which should have the cable attached (mine was disconnected so I basically put it on one of the tabs). Once attached, you should find that your seat belt height will be adjusted as you move the seat forward and back. HTH