Mercury - Tab Extension Harness Assembly 15-Foot Trim Tab Lead 84-898059T15. Overview. Longer boats or wide rigging routes sometimes need extra reach to get trim tab wiring where it needs to go. The Mercury 84-898059T15 Tab Extension Harness Assembly adds fifteen feet of clean, OEM-grade cable so the tab system can be routed neatly from the helm to the actuators or control modules. It keeps the wiring organized, avoids tight stretches, and helps the system respond smoothly without strain on the