if it's the standard cable one rather than the electric one, then the cable goes from the speed to the right angle drive hung off the gbox. Then there is a short piece of inner cable from the right angle drive into the box. There's no oil probs taking off the assembly from the box. You will need circlip pliers to get it off and bck on again. Bit fiddly but it should be poss to see what's going on.
If it's not that type, then I don't have a clue!
Graham