Take out the white plastic cable end from the cluster and clean it up to inspect for damage, if its cracked around the end where it grips the steel inner cable then that's most likely the problem. If it is not cracked then the inner cable itself might be broken. Removal of the cable is not too difficult, but you need to be able to put the car on stands to crawl underneath. When you are reinstalling and removing the cable try not bend it around tight curves too much, other than that, it pretty much strait forward procedure.
If it is the white plastic cable end that is cracked, it can be repaired.... http://www.skylinesaustralia.com/forums/topic/414639-speedo-not-working/#entry6646661
Though I said to use super glue, slow cure epoxy would be better, as this repair failed after 12 months, but with epoxy its still going strong.