If it's the LT77 box, pull the centre seat cover off and away you go! If it's a new cable to be fitted, tie the new one onto the 'box end of the broken one (once detached from drive) and pull through from the engine bay having checked for tiewraps on the chassis rail - then thread in to bulkhead/binnacle and carefully connect to speedo head
It's a fiddly job but worth persevering....