I have an old tandem with SunTour componentry and we cannot figure out how to get the cable out of the grip shift or if it is possible to take it apart. It is a barend/barcon 7 speed shifter which only seems to be shifting 6 speed right now in the rear. Also, if not user serviceable is there any replacement shifter that makes sense? Would prefer to keep it index shifted and also not to have to replace the whole drivetrain.