Mercury - Remote Control Ignition Choke Extension Harness Wiring Extension 84-17829A20. The Mercury Remote Control Ignition Choke Extension Harness is designed to provide additional wiring length between the helm control station and the engine. It is commonly used on larger hulls or dual-station setups where standard harness lengths do not comfortably reach. Using a purpose-built extension harness helps maintain clean rigging and avoids stretching wiring or adding splices that can create stress