The stock Mustang starter relay has 2 large terminals and one small terminal. The housing must be grounded. The battery cable and all the fusible links go on one of the large terminals. The cable to the starter goes on the other large terminal. And the wire from the start terminal of the ignition switch goes on the small terminal. All that is for a pre-1992 starter. If you are using the mini starter the hook-up is slightly different.