The cable on my '88 190E 2.6 was replaced on warranty... I recall because the speedo needle was wiggling causing it to surge on cruise control.

The problem occurred some years later, so I removed the it from both the speedo head and transmission to see if I could pull the cable out to clean and lube. No luck... the cable wouldn't budge from the sheath. It appeared to be sealed with an O-ring at the trans end. I recall the end piece is square, so I rotated it about 90 degrees and carefully reinstalled it, and it's been okay ever since.

Knock on wood!

Duke