Hi Greg @gregsouter
I don’t have the answer myself, but I have asked my local Jaguar Independent Garage to see if they can help. Below is my recollection of how it was described to me.
There is a cable, like a curly data cable, that comes from the rear axle/rear transmission and disappears into the boot area. Within the boot area there are two electronic modules, the smaller of which is the module that translates the pulse coming into it (via that cable) into a voltage. This is then taken as a wire to the dashboard and it is that voltage that gives you the speedo reading.
This means that you’ll only get the voltage out of the electronic module in the boot when the car’s moving and it’s busy translating the pulse from the rear axle.
A thought, do you have cruise control and does it work – the reason for this question is that the cruise control works from the same pulse translation provided by the electronic module in the boot. If cruise control works and the speedo doesn’t then it’s likely that the speedo is broken.
Not sure if the above is of any help, but might lead you to eliminate a few things.