This harness plugs into the dash harness at the firewall on your truck. Common circuits include alternator horn relay, headlights, side marker lights, parking lights, horns, alternator and others, depending on the model year and options on your truck. This wiring harness is for trucks with full factory gauges (not warning lights), and an alternator upgrade that is mounted on the drivers side (i.e. 1972-1984 non-serpentine belt - SI series alternator with internal regulator.