Give this a try!
One end of the speedometer shaft corresponds to the instrument cluster. It differs from the end that fits to the gear box in a small white plastic-cap, a lock bush. After you inserted the drive pin of the speedometer shaft in the corresponding plug at the rear side of the instrument cluster, you have to reposition the lock bush by pressing it hardly in its seat. You will feel or even hear a "click", so the shaft is fastened (sorry, it's not as easy!). The other end of the shaft fits to the gear box. It shows a drive pin too and an union nut. Installation of this side is really simple, but to find the said plug of the gear box is like the search in a labyrinth. Follow the old shaft and you'll succeed. How to pass the bulkhead between the engine- and the passenger compartment, I believe it's selfexplaining.
Good luck, Hajo