mercury 850 wiring harness in eBay Motors | eBay Find mercury 850 wiring harness from a vast selection of eBay Motors. Get great deals on eBay! Mercury Outboard 85HP  SUPPORT, WIRING HARNESS - BOTTOM COWL 37445. Pre-Owned. $15. or Best Offer 20128, Washer, Wiring Harness Clamp Screw Mercury 850, 85 hp, 4CYL,34950XX See more like this. Mercury Outboard 85HP  CLAMP, WIRING Mercury Outboard Wiring Harness The Mercury outboard boat harness (external) is at bottom of this chart. That is the harness that goes from the engine to the control box by the steering wheel. The external harnesses are about 20 feet long.