both ends of the speedo cable are squared, put the dash side in first, and at the gearbox end and twist and push the actual cable until it goes al the way in, when you feel it go into the dash end, spin the end so it lines up with the square ends on the gearbox end, and there you have it

It is a common mistake, and its the most common reason for a speedo not to work after installation.

hope this helps!