Mercury - MP Alert Harness Extension 20-Foot Forward Routing Cable 84-8M0054185. Overview. Some MP Alert installations call for a forward-mounted warning light-especially on larger boats where people may approach from the bow or spend time in front-facing swim areas. The Mercury 84-8M0054185 MP Alert Harness Extension gives you 20 feet of reach to run an MP Alert light toward the front of the boat, making the Moving Prop warning visible from more angles. It plugs directly into the existing MP Al