Mk1 tacho is an inductive pickup, which is different to normal tacho's, that's why running the extra wire from the coil failed to work.

On the tacho you should have a white wire with male/female bullet connectors ?

It is right all Mk1's, 2 or 4 dial, i've seen, have had matching connectors in the main loom. Could be taped in or tucked away out of sight.

Now if you can't find the connectors you can make it work. You will need to find the wire that runs from the ignition switch to the ballast resistor. This wire is then cut and the tacho white wire inserted in series at the cut.