Mercury - Vessel Sensor Extension Harness 5 Foot Sensor Extension Harness 84-899888T21. The Mercury Vessel Sensor Extension Harness (5 Foot) is designed for Mercury diesel applications where existing vessel sensor wiring requires additional reach. It is commonly used during rigging, repower, or retrofit installations when factory sensor leads fall just short of the vessel interface panel, bulkhead pass-through, or sensor mounting location.. The 5-foot length provides a clean extension without ex