I've just had to replace my instrument panel and the support panel behind it as both panels had self destructed over some years in the vehicle and the Army had glued and screwed them together so many times, there was NO plastic left to join to, consequently the whole dash, speedo and instruments were just floating around.
No problem, pulled it all out to swap over, but found the damn speedo cable had fractured (probably because it was moving around so much) right at the plastic connector going into the speedo.
I didn't take pictures, so I've grabbed a pic off the net and have done a PS example...

The speedo was functioning correctly, so I really didn't want to buy a new cable, but I didn't want to get any more grime down into the cable either, nor have to pull this apart again just to replace, so I decided as the cable was now going to be stable after the new dash goes in, why not try to 'bandage' the broken spot and just put it back together again!
So I cleaned the plastic areas with some readily available lacquer thinners to get any grease off and rotated the joint to find where it was meant to seat...

And then grabbed an old tube of two part epoxy putty that you just cut and knead together. I then rolled it on some flat material into a strip about 10cm long (like you use to do with plasticine) and wrapped it like tape around the joint, and again kneaded it a little more to make sure it all pushed together and formed a single piece and also to work it into some of the indents to lock onto any bumps on the plastic joiner and metal cable end, so it wouldn't slip off.
It went hard and totally stabilised the previously broken joint and the cable easily clicked back into the speedo and now works well.
Hope that might be of use to someone.