Wiring Harnesses for Mercury Mariner Outboards Harness lead in Wire for Mercury Verado VST Tank. Connects to the high and low pressure fuel pumps in the VST tank. Mercury Fits Verado Motors Replaces Mercury #: 84 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.