removing the back seat won't do a whole lot of good on a u13, but if memory serves the hole is big enough for a person to fit through. Removing the seat is pretty simple, start with the bottom part (there are 2 12 or 14mm bolts) then the back rest needs a futher 2 bolts and a screw (in the ski port) to be undone and it'll just pop out (push it upwards)
however once your in there I'm not sure what you could do to open it?