Mercury - Inner Helm Extension Harness 60-Foot Extension 84-8M0173575. OverviewThis 60-foot inner-engine extension harness is built for larger triple- and quad-engine boats where the normal harness simply won't reach. When the run has to travel through long rigging channels or wrap around multiple bulkheads, this extension keeps you from stretching the wiring or forcing it into awkward bends. It lets the harness follow an easy, natural path so the controls stay steady. On big offshore rigs, that