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it is possible to upgrade the factory base model
Probably but it's gonna be hugely expen$ive
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Do you think it would be worth it?
No
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Are their more features
6-disks instead of 3
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is the sound noticeably better?
No
Agree with morrjr; you'd have to retrofit the wiring somehow; there's no wiring in the trunk for the (weenie) factory amp.
It's cheaper to change to an aftermarket radio and amp for the door speakers. You can go with a trunk-filling, body panel thudding, neighbor-annoying sub but you don't have to. A small, 10" powered sub is enough unless you're addicted to bass. My Infinity Bass Link makes more than enough for natural sounding bass.