Mercury - Outer Helm Extension Harness 15-Foot Extension 84-8M0159751. OverviewThis 15-foot extension harness gives the outer engines on triple- and quad-engine boats the extra reach they usually need. Long runs and wide transoms can make the factory harness feel tight, and this extension keeps you from forcing sharp bends or stretching the wiring. It lets the harness take a smoother, more relaxed path from the helm to the outer engines. It's a simple add-on that makes big multi-engine layouts m