At the time I made a detailed forum post on this subject, but it seems to have disappeared, and all I have left are the pictures so the details are a little hazy. In short, yes, you can easily fit a MP3 player to a BMW DSP system and have the steering wheel controls work, etc.
You can easily find BMW-aware mp3 players on Ebay – these effectively replace the CD changer – so the front-of-car commands for CD change, skip track, etc, are routed to the new device. The device plugs into the radio module to gain access to the i-bus as well as the audio lines.
So what’s the problem? If your car is fitted with the high-end DSP amp then you’ll find you get no audio output with these devices! That’s because with the DSP setup the amp takes a digital optical signal from the CD changer directly, and the analog inputs are ignored. There’s no easy hack around this – if you try to patch into say the cassette or radio audio you’ll find the controls don’t work – because the system knows what system is active and what control signals should be passed.
The solution is to add an analog-to-digital convertor into the mix, so that the amp can be supplied the digital format it expects.
To install the mp3 device itself you need access to the radio module which is right at the back underneath the padding – you really have to remove everything to access it properly – CD changer, amp, video module, GPS…
With the mp3 in place you have to cut out the analog signal wires from the mp3 devices and attach these to the ADC input. I can dig out the pinouts if anyone needs them.
Then you need to find power for the ADC. There are a few places you can tap it from but as my car was pre-wired for the phone module but it wasn’t fitted I tapped it from the phone connector. This meant it was on its own fuse in case anything went wrong.
You can google the exact phone pinout – but brown and pink are what you need!
With the ADC powered up everything worked perfectly! Sound quality was absolutely fantastic through the DSP.