Factory tach wiring

67Dart273 said

?? I think you'll find the grey wire is one of the senders, oil I think. The tach wire as normally been run out an add-on single wire grommet, separately from the bulkhead connector

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Correct. If they ever did add the tach wire to the standard harness it would likely have went though a previously unused port of the connector commonly referred to a wiper harness connector, just like the reverse lamp wires, and later the neutral safety wire, for examples.
I have a pic of a 67 B'cudas firewall showing that added hole, grommet, and wire saved on a disc here somewhere. I don't recall measurements added to it. So if a general reference would help, I'll hunt it. If my memory serves, the vacuum hose going to a performance indicator instrument went through the firewall in the same location.