Have just changed a rear regulator after one of mine sounded like it had snapped the cable - loud crack and window freely moving up or down. Upon dismantling, the cable was sound but the plastic shutttle (that runs up and down the vertical channel and holds the window clamp) was snapped.
Easier to replace with a unit from a breaker; cost me about £17 with postage and just bolted in of course. Having looked at the cabling I wouldn't even consider a repair kit over a used replacement at that price!
Whether repairing or replacing, leave the cable connected to the motor until you've extracted the old unit through the door. The Haynes manual suggests that you unplug before withdrawing it, but it's got one of those 'M' shaped spring clips locking the connector and hence so much easier to do when you can see it - and you won't lose the locking clip into the depths of the door where inner and outer skin meet.