I tried previously to retrofit the BT module. BMWHK told me that this cannot be done on my car (Z4 35 W/iDrive)
I looked in several places and I was told that I need BMW to code the iDrive once the installation is done... So basically, after 6 months of search, I gave up on the idea and went and bought FISCON. Not really what the OEM BT system does, but the easiest to install and without the need to recode.
If you come across a better way of retrofitting, please share with me the info. Tks