I would go to a different place if you do not want to do this yourself. If your car was fiber optic (Most system, 2003 and 2004 with hifi or bose), you would at least need to rewire between the amp and the HU (while replacing both). But you would not have to rewire the speakers. I have not in my research come across an adapter for the MOST system to be able to use an aftermarket HU with the MOST Amp. But that doesn't apply for you. For your car they should be able to swap the HU and use the existing amp without the need for any extensive wiring. It should just be HU Harness to factory harness without much else.
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2003 Boxster S
2004 Carrera Cab
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