Big props to GROM for making such a great product for bluetooth connectivity.
Originally I bought an aux bluetooth device that plugged into the factory aux port. However, I was unhappy with the sound quality. I tried 2 different aux bluetooth products and both did not sound as good as the radio. Then I came across GROM.... I chose the BT3 (bluetooth/handsfree) that installs in the trunk. The install was painless and only took about an hour. The majority of the time was spent taking out the battery (located in the trunk) and putting it back in. The GROM connects to the car via 2 connectors, no cutting or splicing needed. Once installed, just turned the key and worked perfectly. Best part, it works with the steering wheel controls. Also, my car has the factory bluetooth and when a call comes in, it automatically switches to the car's bluetooth. Can't reccomend the BT3 enough, its worth every penny.
If you are wondering, I've got a 2007 Z4M roadster with factory nav and DSP. Turns out I did not need the DSP conversion kit that GROM offers, so that will be returned.
So if you are looking for great bluetooth and sound. Get a GROM!
https://gromaudio.com/store/all-bmwt.html
YouTube link that shows how easy it is.