Mercury - Inner Helm Extension Harness 70-Foot Extension 84-8M0173576. OverviewThis 70-foot inner-engine extension harness is meant for large triple- and quad-engine boats where the standard harness doesn't come close to reaching the inner engines cleanly. Long routing paths, wide transoms, and deep rigging channels can make shorter runs feel tight or forced. This length lets the wiring relax into a smooth, natural route so the controls stay steady and predictable. On big offshore hulls, that ex