WARNING: This page is about a different car, the 2005 Subaru Forester. However, it is still accessible from the selected car via links, so may be relevant.
Disconnect the ground cable from battery.
Remove the gusset cover.
Remove the tweeter mounting clips.
Fig 1: Removing Tweeter Mounting Clips
Courtesy of SUBARU OF AMERICA, INC.
Disconnect the harness connector and remove tweeter.