Mercury - MP Alert Harness Extension 30-Foot Forward Routing Cable 84-8M0054186. Overview. On larger boats-or layouts with long rigging runs-you may need extra length to bring an MP Alert light forward where swimmers, riders, or guests can see it sooner. The Mercury 84-8M0054186 MP Alert Harness Extension gives you 30 feet of reach so an MP Alert light can be mounted well ahead of the transom. It plugs directly into the MP Alert wiring, extending the system's visibility and ensuring the Moving P