The cable bracket right next to the reversing light wiring looks like it had kinked the cable a bit. This could cause (or maybe already has caused) the cable and inner sleeve to wear out. I know this isn't the most fun job you can do but it might be worth your while to pull the entire speedo cable out and inspect it. I have never heard of the cable spline drive in the gearbox extension housing failing but there's always a first time...