I've got a 2001 Legacy,L,...it has that same socket(no cap...),it
takes a 20-amp fuse...but,this "mod" only works if the car has a
Variable-central-differential-(VCD for short)...
After installing that fuse,a light on the dash will light up,
(FWD),to indicate that the rear wheels aren't being driven-no VCD-
nothing happens.