Mercury - Inner Helm Extension Harness 30-Foot Extension 84-8M0159761. OverviewThis 30-foot inner-engine extension harness is made for boats where the inside engines sit far enough away that the standard harness just won't reach cleanly. On wide transoms or long routing paths, it keeps you from stretching the wiring or forcing sharp corners. Instead, it lets the harness follow a smoother line so the controls feel steady and predictable. It's a simple way to make big multi-engine setups easier to