You need to remove the lock assembly from the backside of the lock, most likely the small sheet metal screw that holds the assembly in place is either missing or stripped out .
Been a while since I have taken mine apart, but I think there are two Phillips head countersunk screws holding a cover on, once you get that off, you can access the lock assembly .
You can remove the screws without removing the rear fender, but it's a pain no matter which way you go .
If the screws have not been removed before, they may be quite difficult to get loosened with the limited amount of room you to work within, with the fender still installed .
Have you tried pushing the button in, this is what releases the seat, not by rotating the lock .