the power conector is for the positive/feed wire
the acc is to the item you wish to turn on and off
the ground is to the earth of the car ,( i.e. a metal part. this is what makes the switch light up when turned on. )
i really should read the whole question before replying.
i should have added;
the acc goes to your washers possitive wire and the other wire from the washers will go to earth.