I have used them (but not on Subarus). Most European imports have a an extra socket in the back tail light assembly. All you have to do (in this country) is connect power to it in conjunction with your front fog lights. I would check the legality of such light as it may be regulated by different states (in some it's plain illegal). The wattage is usually the same as your brake light/bulb.
Some people add it (a small "fog" light with red lens) under the rear bumper and connect wiring to the front fog lights (I have also done so previously). If you don't have front fog light you could add a switch on your dash board.
Some people add it (a small "fog" light with red lens) under the rear bumper and connect wiring to the front fog lights (I have also done so previously). If you don't have front fog light you could add a switch on your dash board.