the two speakers in the back cargo area passenger side, are the subs. there are no subs under the seats (that's an e46 thing I suspect!)
there are many write-ups in the how to section about how to add an amp to the system. I have the twin sub as well, just haven't amped it yet! busy with other things! best of luck! looks sharp!
post details in your signature, it helps others when you have questions!

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2003 X5 3.0i (02/2003) sport package, Sapphire black metallic / Black Dakota Sport seats. compass mirror retrofit, trailer hitch retrofit, Prodigy P2 trailer brake controller installed, PDC retrofit, Lumbar support retrofit. heated rear seats retrofitted. Running boards retrofitted and subwoofer install in progress. 4.8is Brakes installed front and back. Bluetooth TCU installed, Rear Fog lights installed. Retrofitted reclining rear seat backs.