Mercury - SmartCraft Extension Harness 3-Foot 10-Pin Extension 84-8M0138593. Overview. Sometimes a SmartCraft connector just needs a little more reach to land cleanly at the helm or junction box. The Mercury 84-8M0138593 Extension Harness adds a simple three feet of length to any 10-pin SmartCraft connection, giving you the extra slack needed to route cables without stretching or forcing tight bends. It keeps the network clean, organized, and easier to rig in tight spots.. Key Features. 3-Foot E