does it look like its meant to be there there i.e factory, or something that may have been put on after, if its factory, then i dunno, mine dont have it, if it was put on after then it might be a wire for a suppressor, this is normally put on the stop the weeeeeeeeeeeeeeeeeeeeeee noise that comes through the stereo. normally a suppressor is a little silver round unit size of a short fat battery, the body of it will be earthed, and the wire will go to the live from the alternator. (white one on the S12).
either that or it went to somin else that someone has put on, so long as everything works as it should, i wouldnt worry bout it.
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