I asked the same question to my dealer, as I only have the base sound system install. It would require the dealer to install new wiring/fittings, and would cost more than it is worth.
If you are after better sound, as I am in the near future, your best bet would be to have a better sound system installed by professionals.
I am from Australia, and local well known installers offered me an upgrade kit which integrates to the factory head unit. It comprises of:
- 1000W 6-channel amp (powers all four speakers and a sub)
- Required components to integrate the amp to the factory head unit
- 12" Pioneer subwoofer (can't remember the name type, its the kind that doesn't require to be boxed/enclosure)
All for $980AU (approximately $880 US).
For this I get a much better/cleaner/powerful sound - whilst maintaining the factory look!