To connect a stock tachometer, what needs to be done? 
The harness that clips into the rear of the tacho doesn't seem to be working, can I bypass this by running wires from the appropriate connections? I am under the impression one should be to the - on the ignition coil... what else needs wiring (other than the light bulb of course).
 
Thanks guys!

http://rsmotorsport.com.au/files/Tacho_into_MK2.pdf
 
Should help you out a little bit ;) I just followed these instructions to install a ghia cluster with tacho into a GL.
 
Cheers,
 
Rob

Thanks heaps mate, very informative!