Crossy,
Man you got me excited after learning about a 73+ cluster that has the speedometer electronically driven. Like wanting a new toy when you were a kid, I had to learn more about this for my FUEL INJECTION SWAP/ELECTRONIC TRANSMISSION SWAP.
I went to the local CHEVY dealer and asked a mechanic to see what he had in the ALLDATA or MOTORALLDATA computer. Well they had plenty. I will post the wiring schematic for the #18 wire. It goes from the plug from the back of the cluster, to a "Digital Ratio Adaptor Controller" where it terminates. The rest of the wires on this controller go out to the ignition 12 VOLT, vss input, and VSS to ECM signal.
What are you trying to wire up with the electronic cluster?
http://www.studentweb.eku.edu/aaron_cowan4/img004.JPG