Mercury - Remote Control Wiring Harness Ignition and Engine Control 40 Foot 84-896537K40. The Mercury Remote Control 40-Foot Wiring Harness is designed for boats that require extended distance between the helm and the outboard without relying on stacked extensions. It is commonly used on larger hulls where a single, continuous wiring run helps keep ignition and engine control signals clean and dependable. The added length allows wiring to remain relaxed and properly supported throughout the rigg