Having had the dreaded crunching sound and no lift of the rear quarter window on my XK convertible I followed the excellent instructions on this forum( which anyone who has done this knows it's quite involved) and removed the whole assembly. Examining the part it seemed that cables were not broken but merely come off their pulleys I surmised due a lack of lubrication on the pulley track, causing the tension to be released and thus the cables to jump the pulleys. Having priced up the whole assembly from Jaguar(expensive)and found no used parts available I found that there is a repair kit for XK8 but not the XK, for around £30 plus delivery from EBay for that price I decided to risk it and ordered it.
On checking the cable lengths, pulley and 2 springs they appeared to be identical so set to work assembling the whole mechanism using a handy guide on YouTube for the Xk8, which although complicated worked perfectly. One tip I would give to anybody assembling this back in the car would be to bolt the quarter window to the assembly while in the body cavity, it is virtually impossible to try and get the whole mechanism back in otherwise.
Paul