First test to narrow it down - if you have cubby holes behind your seats, you've got the basic system. 6 speakers.
If you have mesh instead of cubby holes, it's the 10 speaker system.
If you have Nav, and DSP settings are available you have the top stereo setup.
If you don't have NAV, you could have either middle or top system.
There is a website somewhere - if you enter your VIN number it gives the original spec sheet.
Oh, and the M stereo was the same as other Z4s
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