I have used the Tunai Firefly for couple of years on my 2010 Z4. Really happy with it, super small, plugs into usb port, auto connect when car starts, plugs and play. Had some rare instances when I had to initiate connection from the phone but otherwise really good experience
TUNAI Firefly LDAC Bluetooth Receiver: High Resolution Wireless Audio Bluetooth 5.0 Adapter with Audiophile USB DAC 3.5mm AUX for Car/Home Stereo Hi Res Music Streaming; Auto On, No Charging Needed https://www.amazon.com/dp/B01HDO66NK/ref=cm_sw_r_cp_api_glt_i_MBFCQYTM9RRGXBN68JQC?_encoding=UTF8&psc=1
TUNAI Firefly Bluetooth Receiver: World’s Smallest Wireless Audio Bluetooth 4.2 Adapter with 3.5mm AUX for Car/Home Stereo Music Streaming; Auto On, No Charging Needed - Car Kit (Black) https://www.amazon.com/dp/B01HDO66NK/ref=cm_sw_r_cp_api_glt_fabc_9BNQS2MEEKYTDA83JSJ2?_encoding=UTF8&psc=1
Your best bet is probably going to be something like a wired FM transmitter hooked up to a bluetooth adapter with the appropriate charger and cables hooked up to your original head unit. There's probably people out there that will modify them for you to add an input, but I wouldn't know where to look or how much they charge, and in my experience a wired FM transmitter sounds fine.
Are these adapters (GROM, Parrot) in the $150 range ? If so, that's not as bad as I was thinking....
Budget option is something like this: https://smile.amazon.com/Yomikoo-Interface-Connector-2003-2011-2006-2010/dp/B01EUZ90II/ref=sr_1_1?qid=1568856305&refinements=p_n_feature_keywords_browse-bin%3A3822502011&s=electronics&sa-no-redirect=1&sr=1-1&th=1
Get one of these and plug it into the USB connection: https://smile.amazon.com/Firefly-Bluetooth-Receiver-Smallest-Streaming/dp/B01HDO66NK/ref=sr_1_1?keywords=tunai&qid=1568856552&s=electronics&sr=1-1
Do NOT connect the 1/8" cable to your phone though. Connect it to the Bluetooth receiver. Looks like you'll need a 1/8" female to 1/8" female "coupler" too. This seems very clutzy the more I think about it though.....
Here's what I have. Works great.
https://www.amazon.com/gp/product/B01HDO66NK/
https://www.amazon.com/gp/product/B088CYWT5H/
I just wish that power port in the armrest didn't turn off when you turn off the engine. Kind of a strange decision on BMW's part, but there's nothing you can do to change that, I guess.
Plug one end to USB, other to Aux, connect to firefly from phone, enjoy nice crisp aux audio. Sounds much better than FM transmitter and you still have access to cigarette port
I have this one.
https://www.amazon.com/dp/B01HDO66NK
Plugged into this phone charger.
https://www.amazon.com/dp/B088CYWT5H/
I was a little worried that it wouldn't fit in the center console without hitting the lid, but it works good.