Basically the drive axle turns the insert, which turns the key, which turns the gears in the bearing holder, which turns the cable, which turns the speedo input shaft.
With the track off the ground (jack up the rear) have someone else turn the track while you simply work backwards from the speedo to see what all doesn't turn. The last thing that doesn't turn is likely the culprit. You can also work from the axle end and see the first thing that doesn't turn. Be aware that there may be more than one problem.
Based on what you have said I am guessing that either the cable is broken, the insert is shot, or the speedo itself is not working properly. You can stick the key in the speedo and turn it to check to see if it registers.