Mercury - MP Alert Harness Extension 10-Foot Forward Routing Cable 84-8M0054183. Overview. Some MP Alert installations require a forward-mounted light-especially on boats where swimmers or riders might approach from the bow or forward quarters. The Mercury 84-8M0054183 MP Alert Harness Extension gives you a clean, 10-foot extension to route an MP Alert light toward the front of the boat. It plugs directly into the existing MP Alert wiring and keeps the system's warnings visible well beyond the t