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If you have only 6 speakers, then that would appear to be a standard system, with no standalone amplifier in the trunk. The premium system, with ten speakers does have an amplifier in the trunk. (MY 2004 and on)
So that would imply that the original head unit that you want to use is U/S and won't drive your new amplifier.
Is all that so, or do you have an amplifier in the trunk?