Another picture showing (kind of) the general location of the wire. You're looking at the auxillary water pump and the wire running up toward the coolant overflow tank. It appears to connect, at one end, to the voltage regulator on the back of the alternator.

The other end seems to go under the battery toward the firewall.

Any help?

Color - looks like the same as the car - beige. Same color as the wires at the wire harness/connector on the top of the passenger front wheel well.

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83 300d - 390k