Ah; that makes sense seeing the other unit without the SIM slot and aerial connector. I thought the aerial might be shared but I didn't realise that BT in the 4H cars used the phone's aerial. A backward step from the early A8s that used a 'SIM in the arm' and an external GSM aerial.
So it looks like at least a 'control head' swap, an aerial run and an amplifier. And that's before discovering whether the MMI will recognise those changes.
Julian