CheapnNasty said:
There is a plug that goes straight from the back of the tape deck to the ipod. Can't remember what it's called. It basically plugs where there shuttle plugs into the back of the deck with an ipod connector on the other end.
I did some research on this recently.
On the latest models this is all digital and you need A2D converters and stuff. Don't know much about that though.
On later e46 models you have the auxiliary input, which is effectively the usual three wire stereo connection (ground, left, right) with a resistor between the two signal lines (so the HU can "detect" the cable) and two caps (to block DC signals). You can buy the cable from BMW (its bloody expensive) or make our own for less than R100. I think this is what you're talking about.
But on the older head units (before mid 2003) you have no aux cable, but you still have a normal three-wire stereo cable running from the shuttle to the back of the radio. That's not the problem though. The problem is that the head unit will only use this input if there is a changer in the car. The head unit and the changer communicates over the ibus.
Some people burn a cd with silence, play that in the changer (so the changer keeps the channels open), then feed a signal from an ipod into the head unit. In the computer business we call that a "kludge", a solution that works but is really quite crude and horrible.
The clean solution is to put something on the line that pretends to be a changer. The Dension gateway I have in my car works like that: it tells the head unit that its playing disk 5 track 99, and feeds it the audio signal. Unfortunately the dension products are also quite expensive.
Now the next question you probably want answered is whether your head unit is new enough to work with the aux cable. Do this:
With the ignition off, turn on the radio. Hold the "m" key for ~ 10 seconds, until the unit serial number is displayed. Now press +. The week/year build of the headunit and the software version is displayed.
You need at least version 50 if I recall. If you have an older head unit, you cannot fit the auxiliary cable. You can either get a new head unit, get a tape adapter, get an FM transmitter, use the silent-CD kludge, or get a dension or ice-link.
Finally, this also fascinates me:
http://www.reslers.de/IBUS/
With a little mini-itx board and a minimal linux distro some pretty impressive things should be possible. A project for another day.
