Mercury - Inner Helm Extension Harness 20-Foot Extension 84-8M0159759. OverviewThis 20-foot inner-engine extension harness gives you extra reach for the inside engines on triple- and quad-engine boats. When the standard harness won't quite make the run-or the routing path ducks around bulkheads-this length keeps you from stretching the wiring or forcing tight bends. It lets the harness run in a smoother line so the controls feel steady. On larger multi-engine rigs, that extra room makes rigging