78 SC 3.0 to 2.7 downgrade and tach wiring

[julahti]

My '78 SC had 3.0 replace with '77 2.7 engine (great...) when I bought it and looks like someone didn't figure out how to re-wire the tach. Neither do I.

The tach has three connectors side by side. + and - on the sides, and the one in the middle has a red wire and a red/white wires coming to it. What are they anyway? Why two wires?

Where in the engine bay I should be looking for this red/white wire, which I suppose is the one coming from the engine bay? I have looked for a wiring diagram for 78 SC but no success so far. I thought the tack wire should be black/purple?

I jumped the wire straight from the coil to the tach, and the tach works.