A 03 has the MOST bus so any normal upgrades won't work. You will need to basically bypass the fiber cable by removing the OEM head unit and amp. I put an Alpine HU/Alpine Amp and treated it like a new install and ran new wiring. I cut the OEM amp wiring and spliced in the new Amp so I could use the OEM speaker wiring. All work great. There is a place called CAI (
https://cai-store.com/) that sells the equipment and most importantly wiring harnesses. The cost is a little high but the harnesses and support make it very easy to do.
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