WARNING: This page is about a different car, the 2004 Subaru Outback, 2004 Subaru Legacy, and 2004 Subaru Baja. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect ground cable from battery.
Disconnect connector of high-mounted stop light from body harness.
Remove bolts, then detach high-mounted stop light assembly.