hmm, I'd swear I replied to this last night. I was able to determine that the X4S is the correct one and that the X2S is no longer made.

Found it for $29.96 at SoundDomain, plus $4.95 Shipping.

Crutchfied has it at $34.95 + $4.95 S&H.
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Rio Car 30gb
2000 Boxster S, <img src="/ubbthreads/images/graemlins/cool.gif" alt="" />