for the 80 450SEL it ws a matter of removing the panel above the driver's legs and unscrewing the speedo cable from the back and then giving the instrument cluster a nudge. It's held in by a rubber seal. You can use a hook type tool, but my way is easy. If your just removing the cable you shouldn't have to remove the cluster.
As far as the cable itself I removed the cable to the service counter (cable is about 18 to 24 inches long). I then treated the cable to a teflon based lubricant and stood it on end for a few minutes to let it drain down. Then reinstall.
So far so good. There's no downside, upside is it may work.