Mercury - MP Alert Harness Extension 50-Foot Forward Routing Cable 84-8M0054187. Overview. Some boats need the MP Alert warning lights positioned far forward-well beyond the reach of standard rigging lengths. The Mercury 84-8M0054187 MP Alert Harness Extension gives you 50 feet of cable, making it the longest forward-routing option available for MP Alert installations. With this harness, you can mount a Moving Prop Alert light near the bow or along extended deck areas so swimmers, guests, and ri