I dont know anything about the Legacy, but I can tell you that many times
you
have to access the taillight fixture from inside the car.  On older cars
there were
often external screws that made replacement of bulbs and lenses easy, but
now
rear access is more traditional.

You may have to remove some nuts from studs and remove the fixture so that
you can replace the lens.

http://tinyurl.com/hz8rl

This may help.