The premium package went through a couple evolutions, but basically there was an external amp which was integrated with the upgraded head unit, and the car had 2 (or more in later models) additional speakers.
I'm going to go out on a limb here and suggest that the guy who worked on your engine only put in a new engine harness not replacing all 7 (or however many) harnesses there are threaded all across the car.
Look in the TEWD and look in your car for an amplifier.
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