It's shame that such an expensive part doesn't work properly and the manufacturer does not solve this common problem. I would do the followings:
First: get off the gearstick gaitor and check the sleeve: when you lift up the sleeve, the bottom of the reverse-lock sleeve must be higher than the top of the plastic gear lever housing otherwise you cannot (pre)select reverse. If this occurs then:
1, determine the reason of the problem: Is it caused by the too short sleeve groove or the wrong position of the gearknob.
2, if it's caused by the too short groove (that doesn't let to lift up the sleeve enough high), then: you must cut / grind a bit of the sleeve bottom
3, If the gearknob sits too low: the gearknob is threaded -> put a washer onto the top of the stick -> you won't be able to screw on fully the gearknob -> you will be able to lift up the sleeve higher (in this case you should use high strength thread locker)
If the sleeve is fine, then:
1, check the rubber gaitor (cube shaped) at the gearbox side: whether when you(r friend) select(s) reverse (in the passenger cabin) the gaitor enables taking up the right position of the selector arm or the gaitor seizes between the gearlinkage and the selector arm making impossible to select reverse.
2, get off the cube-shaped rubber gaitor at the gearbox side and check: whether the (long) gearlinkage hits the gearbox housing when it's in reverse or the head of the clevis bar touches the selector arm. If this occurs, you have to grind the parts a bit.
3, disconnect the elbow-joint at the gearbox side. Select all gear by moving the selector arm by hand. In all gear you have to check the clearance of the small gear linkage: Can you turn around the small gear linkage around its axis? If you cannot then the diameter of the bigger ball is wrong. If you can then you have to realign the clevis bar... that can be a real pain.
Hope this helps.
