When I bought my car, mine didn't work and I'd never seen it move.
I bought an Ignition Failure Sensor second hand from someone wrecking an engine, although you can order them new from RiceRocket in the UK too.
Ignition Failure Sensors have a reputation for dieing and then you don't have a working tacho anymore.
The actual sensor sits just behind the radiator fill cap, below the piping from your air intake to the throttle body.
Replacing it is as simple as removing your intake pipe, unbolting the sensor and then there is a piece of wire in a U shape that sits over the plug - you have to get a screw driver under the wire and pop the wire off, then the plug slides straight off the sensor. Plug in your new sensor, snap the U shaped wire back on and bolt it back onto the block, replace your air intake and you're done.
I'm not saying that the IFS is the only thing that stops your tacho working, but for roughly $60 and half an hour's work, you might as well try this before you send it off to the auto electricians for $$$ of fault finding...
Hope this helps someone!
