the white wire is the ground side of the starter relay which must go to a ground referrence (mercury switch) typically if the engine is tilted UP the mercury contacts will be open and ground reference is lost and starting will be impossible.
the vacum (cut-out) switch where your mercury switch meets is itself normally NOT grounded leaving the mercury switch to do it’s job by itself.
the vacum switch is there to kill 50% of the ignition has described in the manual and since you would not be turning the key to START during that event the 2 wires would not interfeer between themselves.
they work together.. the Mercury switch PREVENTS starting during tilt up The vacum switch kills 50% of the ignition by grounding the lower plug (see manual) see diagram from MarineTech site
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