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If the wire was a small red wire it was the idiot light wire. If the wire was blue then it was the case ground wire for the alternator. If the rest of the wiring is in OK shape then you can buy the idividual repair wires for this from a vender like IPD. Of course you can also make your own. The ground wire runs from the case of the alternator to the bracket it mounts in. If the small red wire or the wire that goes to the oil sensor just behind the alternator are bad you can run the new wires around the side of the engine compartment and along the rear firewall to their connection at the gray connector at the rear of the motor. Make sure to use wire that is the same size as original. I recommend checking the large red alternator wire where it runs under the crankshaft pulley if this is
happening to the small wires. If the large red wire grounds it could cause all
kinds of expensive problems. The complete wiring harness is also available for
a bit of money.

Dave 82 242ti 273k mi.