disconnecting the speedo cable from the cluster has nothing to do with unscrewing
it is held on by a clip/tab, if u can manage to get your hand behind the cluster, run your fingers onto the top of where the speedo cable attatches to the cluster and push down on the tab, it should be a clip, then pull the cluster out a bit, when the speedo cable comes off it will give u more play/room to unplug the wiring.