This might be a good time for a little instrument cluster removal warning.
On clusters with cable type mechanical speedos, you must disconnect the speedo cable from beneath before pulling the cluster. Many will pull the cluster just far enough to reach behind to unscrew the cable. Doing this stretches the cable housing. When this happens the cable doesn't have enoug length to reach one or the other end.
The way you describe the problem sounds like this has happened.
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician