Firstly , it’s an electronic speedo so no cables involved.
Speedo not working is a common problem and likely to be caused by a bad connection somewhere along the line. Is also possible that a diode on the instrument cluster has failed and or the speedo pick up in the gearbox is contaminated but firstly I’d check the electrical connections. These are at both the instrument cluster end and underneath the car at the offside rear end just behind the rear wheel (there should be three connectors in a line, the one closest to the wheel is the speedo one (well it is on my car) disconnect it and spray with switch cleaner).
To clean the connection at the instrument cluster. You need to remove the top half of the instrument binnacle. Disconnect the battery. Undo the screws at the bottom of the instrument binnacle. Remove the steering wheel (need two M6 screws I think to pull the boss off) and remove a couple of screws from the panel directly underneath the where the steering wheel enters the dash; just makes it easier to ease the binnacle off.
Then carefully prise off the top part of the binnacle taking care not to yank too much on the cables locating to the bank of switches on both sides. These cables will need to be disconnected so you can remove the unit completely. Also, remove the two air vent covers as it helps to remove the unit without causing damage. Once off you’ll see the main connector on the LHS of the speedo – it’s a large (ish) white connector block. Disconnect and again spray with switch cleaner.
First time I took mine apart it took about an hour. But I find I need to clean the connections every couple of months or so and hence I’ve got it down to 5 mins !
If the speedo doesn’t spring into life then your looking at either the pick up being dirty and needs to be removed and cleaned (not a difficult job but just a sod to get to if you haven’t got access to a ramp) or the diode on the cluster being blown.
But my money is on bad connections !
Good luck
Paul