Mercury - Outer Helm Extension Harness 25-Foot Extension 84-8M0159753. OverviewThis 25-foot outer helm extension harness gives you the extra distance needed to reach the outer engines on wide-beam triple- and quad-engine boats. When the standard harness isn't long enough-and the rigging path winds around bulkheads or across a wide transom-this extension keeps you from pulling the wiring tight. It lets the harness run in a smoother, more natural line, making the whole setup feel calmer and easier