I believe the hardest part will be getting to the cable's attachment near the front bow. The next hard part will be threading the wire through the fabric sleeve along the top of the door opening. I suspect that without the cable you won't notice much difference. Have you driven the car at highway speeds with the broken cable? Worst case scenario is the seal along the top of the window will be open up a little while driving. If it does not you might want to wait intil it is time for a new top.
Is there a write-up anywhere on the steps to replacing a cable with the top already attached to the frame? If I don't have to remove the canvas to replace it, I might as well, but if I have to, I might just live with it. No I haven't had this top at highway speeds yet so I dont know if it will be noticable.
FWIW, I been driving with no cables at all with no problems since the day I got the car.
At most, you may have to loosen the remaining one so as to not have it pulling the top over to that side or something. Beyond that, I've never quite figured out what their actual purpose was as things seem to work just fine without them. :shrug:
Use a coat hangar to fish the wire through the pocket shown. Connect the front cable loop. I used a steel rivet. At back end fish down through the top frame as depicted and down to the knurled adjuster and cable stop. The tension should be adjusted so as to just compress the spring a little. If you tighten it too tight, you'll wind up pulling the front fastener out or breaking the cable.