Mercury - Outer Helm Extension Harness 40-Foot Extension 84-8M0159756. OverviewThis 40-foot extension harness is built for wide transoms and long rigging runs on triple- and quad-engine boats. When the outer engines sit way outboard and the factory harness just won't reach comfortably, this piece gives you the distance you need without stretching or forcing the wiring. It lets the harness follow a smoother path so everything feels calmer at the helm. On big offshore rigs, that extra room makes r