Hi,
Just checked the wiring diagram and theres 2 of the wires go directly to the clocks (WHITE & BLUE/YELLOW).
Before you go chopping wires make sure ALL the connections are good on the multiplugs, if poss check the backs of the multiplugs where the wires enter them too (you want to be looking for corrosion or broken wires).
Im fairly certain that the WHITE wire is the one that would be causing the fault as it is this wire that gives the speedo its necessary signal. One piece of wire should be ok, but dont forget you also need to wire it to the C.D.I too.
If you do go down the route of chopping the wires, make sure you cut them far on enough from the plugs so that you COULD rejoin them if it is not this that is at fault.
One thing I would be tempted to do first is disconnect the plugs from the clocks, speed sensor & C.D.I unit and test the wire for continuity from one end to the other and also check that the loom does not have a short to earth by twisting & pulling at the loom from the speed sensor end back to the clock plug.