This took me about a week to finally get working after getting the parts in. As some background, I have a 2008 135i with the factory iPod/USB/Aux option. I previously had a Parrot Tablet in order to have bluetooth and bluetooth audio functionality but was tired of it blocking my center vent.
I had read on E90Post a few threads of bluetooth retrofitting, but most involved the use of a TCU which the car didn't have. I went and bought the TCU as well as the necessary bluetooth antenna and microphone on eBay and installed them since my car was prewired for bluetooth. This involves removing the lights/sunroof controller by depressing the two tabs at the front using a screwdriver. Once it's out, you remove the dummy mic clip and insert your microphone and plug the connector in.
After installing the mic, I installed the bluetooth antenna. It's located in the kick panel under the steering wheel. Remove the torx screws and pull it down and near the back left is a wrapped connector that fits the antenna. Connect and you're good to go. The antenna has a slot to clip in and I secured using double sided foam tape.
Finally, at the rear of the car is the MULF2 mounted with an empty spot for the TCU. If you already have the MULF2, don't waste your time and money to get a TCU since the MULF2 can serve the same purpose (and in fact, I couldn't ever get the TCU to work properly with the MULF2 installed). If you're using your MULF2, take the TCU prewiring and remove the clip on the white/purple connector so you have a simple SMA connector. Plug it into the MULF2 empty terminal.
Next, you need to take the TCU connector and remove it from the connector bracket and remove pins 1 and 19 (microphone pins) and move into the connector already attached to the MULF2. Put old pin 1 and 19 into MULF2 connector pin 1 and 19, respectively. This allows your MULF2 to have microphone wiring.
Lastly, under the dashboard next to the fuse box you need to rewire the microphone by moving 3 pins. Please see the linked guides that inspired me below.
Once you've done all this, simply FA_WRITE $644 to the CAS and NFRM, then SG_RESET the 2URAD and 2MULF. My car needed to sleep overnight, but the next day bluetooth worked.
Guides I used that will be useful:
Installing the microphone and antenna (and TCU if you want to go that route)
Rewiring the microphone under the dashboard next to fuse box
Moving microphone pins from TCU connector to MULF2 connector