Mercury - Inner Helm Extension Harness 25-Foot Extension 84-8M0159760. OverviewThis 25-foot inner-engine extension harness gives you the extra room needed to reach the inside engines on triple- and quad-engine boats. When the standard harness comes up short-or the rigging path winds through bulkheads or tight corners-this extension keeps things from getting stretched or pinched. It lets the wiring settle into a smoother route, which helps the controls feel more natural. On larger multi-engine ri