W124's are easy to get the cluster out. Just undo the very lower dashboard trim (near your feet) then reach up to the back of the cluster and push it out. It is only an interference fit with no screws or bolts holding it in place.
Once partially out, disconnect the speedo drive and then you can pull it out further, check all the bulbs (including the one that acts as a fuse) and if that fails, clean or replace the rheostat.