I experienced exactly the same problem with my 99 Legacy SW. The dealer replaced the switch, to no avail.
I had a run-in with a snowbank this past winter, and the drivers door had to be replaced.
I haven't had any problems with the PW since then. I think they used my old door panels, but the motor unit may have come with the door (it was recycled).
I don't recommend that as your solution though.