Oddly it's one of the things I'm hoping to look at over the easter break!There are 3 ways to connect it in:
- Use one of those old cd-tape adapters that will just plug into the IPOD. Quality will be awfull though!
- Use an FM transmitter (modulator) such as the iTrip - though I use a Sandisk player so I use a separate FM transmitter instead. Quality is good, but technically still below "cd quality" - cd is 44100 samples per second, FM is 32000 samples per second (ish)
- Sneak it in via the CD player connection
With the 3rd one, the 2 things to consider are the audio side and the control side.
The Audio side is slightly easier in theory - there are 4 wires that go from the cd player into the back of the head unit - if you disconnect these you should be able to wire these to a lead from your IPOD - though you could blow up one or the other as I haven't found any specs for the electrical signals. I'm planning to borrow some test gear from work to try measuring it and see what i find. If it works, I'm planning on fitting a switch that will allow me to toggle the audio input between a jack for my mp3 player, and the original cd player instead.
To make that lot work, you just leave a cd in the player and the head unit will think it's playing the cd, but the audio will actually come from your mp3 player.
The better solution would be to use the Control interface as well, thereby removing the cd player from the equation.
The control connection in a DII uses a format called the IBUS - this was installed by BMW, but was originally designed by Saab I think. In a beemer (and other cars as well) it's actually a network that links all the computer bits together including the audio gear - same as the (Canbus?) in a D3, but just a different make.
In a DII it's only used for the Alpine HIFI gear though. There are a few people out there making bits that will allow you to use the IBUS for something else, but I don't know if you can buy a box "of the shelf" that will just allow an IPOD to emulate a CD player.
You can find more details from http://www.reslers.de/IBUS/index.html and http://autos.groups.yahoo.com/group/HackTheIBus though.
If you do go this route, it will be very interesting to hear how you get on! I've got a few plans of my own, but with the new family member finding the time is pretty hard!