Mercury - Harness Extension (10-Pin) 6-Foot Instrument Drop Extension 84-8M0058668. Overview. Instrument drops can come up short fast, especially on wider consoles or when rerouting controls during a repower. The Mercury 84-8M0058668 Harness Extension gives you six extra feet on a 10-pin SmartCraft instrument drop, making it easier to reach gauges, controls, or helm devices without stretching the original harness. It installs cleanly and keeps the network tidy, even in tight or unusually shaped