Electrical System Diagnostic Plug / Test Port

[Mordred]

I can tell you there is one wire that goes from the plug to the 8 way connector above the fusebox. (i think blue or blue green from memory and may have been originally for the rear defrost as in the diagram 5).  You can link it to the one from the coil 13 and then extend from the fusebox connector (it is the wire that ends at that connector and is blank on the other side) and use it for a tacho on the dash or you might be able to use it for a pressure or temp sender.  Works a treat.

Adrian