Now that's an issue I haven't seen forever.
On the trans side you have a gear and adapter. This is what the cable screws onto at the bottom. The o/d of the gear shaft will ride in a seal. Both can wear out. Over time, fluid can be pushed up the cable into the back of your cluster. I don't remember if the cable itself had a seal also. My memory says no but its been decades for me. The seal itself may not be replaceable and i would replace the gear and housing just because of age.
Driven gear. This is a generic picture I found. Don't order off my picture. I don't know your year or how many teeth are on your gear and would hate for you do get it wrong.. I would also replace the speedo cable.
If you have this design, you see the spirals on the driven gear shaft. That's suppose to push oil backwards. It also looks like there's an oring on the shaft itself. In my experience, there is also a seal on the end of the adapter. Like i said, it's been decades since i worked on one.
