As mentioned in the other post, there are three fine bolts which hold the tacho onto the circuit board. They also provide the electrical signals for it. These are identified on the circuit board as IG- IG+ and SE. Remove them and the tacho will fall out the front ( carefull!).
If someone can tell me how to post pictures I will - perhaps a separate tutorial on the capacitor replacement might help a few people....
If you've got access to an automotive multimeter with a tacho setting use the diag plug first to make sure you're getting a signal - but it is most likely the tacho itself ( got a spare?), or possibly the capacitor(s) which are not too difficult to replace if you have some basic soldering skills.
Look for the two little blue cylinders sitting on the tacho circuit board. Bad ones often look like they are very dirty.
Chris