A subwoofer, not plural. It's in the spare tire well and required the reversal of the tire mount plus spacers on the trunk floor. The cupholders have no cover, there are tweeters in the sail panels--those are both also right.
I have an 03.5 and have ridden in a friend's 02 and I have seen a couple of other differences, but I don't know at what time the changeover occurred:
1) She has a very awkward looking red light sticking up in her gauge cluster in front of the glass--is that an alarm-armed light or something?
2) It seems the earlier P5's may have had a shift knob more similar to the sedans, with a round knob dimpled around the edges and a shift pattern on top. Mine has a goose-head shaped leatherette knob with an embossed shift pattern.
Like I said, I don't know what these differences signify (nor, actually, do I really know if they are different--I could be totally wrong), but these are just things I've seen.