Get an audio integrations box. Very clean, very OEMish. And exponentially better bass than a dinky under the seat 8 incher.
AI is even running a group buy right now to save ya some $$$
http://forums.nasioc.com/forums/show....php?t=2450558
-edit. wait a tick.. is the 5 door impreza i different than the wrx/sti? I forget, don't have enough cares to look it up. lol sigh