Mercury - Inner Helm Extension Harness 15-Foot Extension 84-8M0159758. OverviewThis 15-foot inner-engine extension harness gives you the extra reach you often need on triple- and quad-engine boats. Sometimes the factory harness just won't make the run cleanly-especially when the path dips around bulkheads or stretches across a wide rigging bay. This extension keeps you from forcing tight bends or stretching the wiring. It lets the harness settle into a smoother route so the controls feel normal