nobody said it was impossible.... only (almost) impossible on the REAL Premium Audio/Top Hifi (thus Carver) system due to the fact that the Premium Audio amp (or Carver amp or however you want to call it) needs a digital audio inputsignal.
Premium Audio = Top Hifi = Carver 10 Ohm subwoofers
I installed only last week another headunit in a Z4 with the Hifi/10 speaker + amp setup. That is indeed just plug and play.
Can you confirm you have the "real" Premium Audio system (thus with 10 Ohm Carver subwoofers and not the "normal" 2 Ohm subwoofers?) You could check if you had the DSP setting on the OEM system.