I don't have my Boxster book here at work with me. So I really can't check the diagrams for you.
But you will notice that the sound bar end only has 4 pins. Those pins should be ground and a positive for the left set and right set of speakers. With the soundbar out, and the speakers unplugged a DVOM will tell you which wires go to what. At that point I would say wire them directly to your new head unit. If you need replacement speakers, Pioneer, JBL and few others make speakers that fit the sound bar.
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