The tach wire is a single brown wire that comes through the firewall in that area. Could also be a TCS wire. Follow it back on the inside of the car. If it runs up to the top of the gauge cluster and looks like it goes into the connector it's probably the tach wire. If it runs down below the steering column the it's probably a TCS wire.
What harness are the orange, purple and red wires in?
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Hank
70 Chevelle SS396(454), M20, 3.73
My Chevelle
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