I think ahpook is correct it is a Ground, but to find out where it lives on the PCB board's
I would guess that it would be a Black wire that goes to the Switch Board.
Pickup Cable (Gray) and Ground (Black) coming from the Pickups Area.. then all go to the Switch,
Look at the soilder joins and see if some thing is attach, if some thing is missing then that MIGHT be it but keep looking, with hope only one thing would be missing.