* Consult a wiring diagram: A factory wiring diagram specific to your 1978 Trans Am's VIN is the most reliable source. These can sometimes be found online through Pontiac enthusiast forums or websites specializing in vintage car repair manuals. A Haynes or Chilton manual for that year and model might also help, but factory diagrams are preferred.
* Trace the wire: Carefully trace the wire from the tachometer itself back to its source (likely the ignition system). This requires some automotive electrical knowledge and patience.
* Use a multimeter: With the ignition on, use a multimeter to test the voltage on various wires near the ignition system until you find one that pulses with the engine RPM. Be extremely cautious when working with automotive electrical systems. Incorrectly probing wires can damage your car's electronics or cause injury.
Without a specific wiring diagram, any color mentioned would be unreliable and potentially dangerous to rely on.
