Just pulled my W124 cluster a few weeks ago. The Haynes manual states that the cable should be disconnected from the transmission...I don't believe that is necessary if your cable is OEM.
There is enough cable length that you should be able to locate the cable down by the footwell and encourage maybe an inch or so of travel by pulling the slack upwards towards the gauge cluster. That should give you enough cable to be able to pull the cluster out and over a bit to gain access to say, bulbs.
Besides, the speedo lube is pretty nasty stuff, and when you start disconnecting that stuff, you run the risk of getting it on your hands, clothes, on that Palomino colored interior, etc., etc.