Mercury - Vessel Sensor Extension Harness 15 Foot Sensor Extension Harness 84-899888T23. The Mercury Vessel Sensor Extension Harness (15 Foot) is designed for Mercury diesel installations where sensor wiring must travel longer distances between vessel sensors, interface modules, and control components. It is commonly used on larger boats where standard sensor leads are not long enough to route cleanly through engine rooms or rigging passages.. The 15-foot length allows relaxed, strain-free routi