Mercury - Tab Extension Harness Assembly 30-Foot Trim Tab Lead 84-898059T30. Overview. Extra-long boats and complex rigging paths often need far more reach than the factory trim tab harness provides. The Mercury 84-898059T30 Tab Extension Harness Assembly adds a full thirty feet of length, giving you the room to run the wiring cleanly from helm to transom-no stretching, no tight bends, and no awkward detours. It keeps the system responsive and gives the rig a cleaner, more professional finish..