If you're feeling confident enough with electrics etc.
Black Wire - Tacho Wire
Test this in the engine bay when the car is running, you should get a signal between 1 - 6v
If that is fine, then you test it under the dash, you will find this on the Main Harness to the right of the accelerator. (Don't be scared, there's ALOT of wires here)
If that is ALSO fine, check the instrument cluster harness.. If that is ALSO fine..
The last thing is your cluster..
SuFz