Mercury - Outer Helm Extension Harness 50-Foot Extension 84-8M0173571. OverviewThis 50-foot extension harness is made for big wide-beam boats where the outer engines sit a long way from the helm. When the standard harness doesn't even come close-or the routing path snakes around bulkheads and long rigging channels-this extension gives you the reach you need without forcing anything. It lets the wiring settle into a smooth, natural run so the controls stay steady. On large triple- or quad-engine