unknown wire?? help!!!!

hello, well i was working on my 1993 nissan sentra SE-R(installing a alternator) which i wasnt the one who took it out so i dont know what one wire is.. one wire has a rubber peice on it and it bolts to the back of the alternator, and then the connector goes into the back of the alternator.. but then there is another wire.. like a ground wire or something... could someone tell me what this wire is and where it goes??? thanks in advance...