Where is the brake line located for the back passenger side on a 2003 Legacy L Sedan? I can see the fluid dripping from a plastic cover below the door in front of the wheel.

I'm not completely sure on that year but earlier years the tradition was the lines ran from under the rear seat above the fuel tank to each rear wheel, at least on wagons though I think sedans might be the same way. If you life the rear seat on the passenger side you might see like a plastic sticky piece that if you life will reveal where the brake lines go from inside the cabin to under the vehicle.

Most likely corroded out brake lines.  But do as P75 says and you will see the two brake lines going through the floor under the rear passenger side seat.

Yep, both rear wheel ones go through there, they go down, go to a mounting/junction block, then one goes to rt wheel, one to the left.

 

Probably the other one is ready to go to. My drivers side rear line went out, nothing usable all the way over to the connection at the control arm.

 

I patched in new up top.