The yellow wire on a 2000 Crown Victoria P71 alternator runs towards the firewall to provide a sensing voltage to the voltage regulator. This sensing voltage is used by the voltage regulator to control the output voltage of the alternator.
The other two wires that go with the harness are the battery terminal and the ground terminal. The battery terminal provides power to the alternator, while the ground terminal connects the alternator to the chassis of the vehicle.
