WARNING: This page is about a different car, the 2001 Subaru Outback and 2001 Subaru Legacy. However, it is still accessible from the selected car via links, so may be relevant.
Remove cap (a) by prying on the edge with a screwdriver.
Remove screws (b) and then detach cover (c).
Remove screws (d) and then detach high-mounted stop light while disconnecting connector.