You can do a lot with a sound system without occupying all of the precious cargo space... These are serious competition systems, don't forget.
I'd be happy with a little more output and a cleaner sound. As such, I'll probably either replace the head unit or get line-out convertors. Possibly an amp with 'em built-in even. Then, just upgrade the speakers.
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01SE 6cyl 5spd SolrYelow