If your car has HiFi, it has two 8" - one under each seat.
I don't think you can get a 2011 M3 with Stereo audio (Technic?). Stereo audio systems have 6.5 under the seat (i 8" holes with large plates) and no amp at all, and everyone who gets a car with Stereo audio hates it.
But even with HiFi, two 8" woofers in a vert have a helluva time compressing the air enough to create sound pressure.
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Kawasaki KE100