Will,
First I thought it may be the angle drive, but if that were a problem one would assume it wouldn't turn or at least turn slowly so shouldn't bust up the cable.
That only leaves two scenarios;
1. The cable routing has a tight radius and is binding somewhere.
2. Your speedo has an internal problem.
I expect even though the problem or binding may be at the far end, the ZF drive would twist the cable up and it would have a tendency to break at the drive end.