I'm going through my GS wiring now and found the same wire along with a off-white w/ brown tracer in it, that goes to nothing also.
The blue had a black tracer and went to nothing on both ends. The off-white/brown tracer was on the other side of the bilge switch and ran to nothing on the bilge end. (I think it's meant for a bilge pump with a float switch.)
I want to say there is roughly 9 wires total in the white plug. Goes to nav lights (power & ground), main (red) power, bilge pump (power & ground), ground wire, fuel guage sender (both feed & ground)