Alright, so let's begin.
In the pursuit of a solution to playing music from my phone in the e34 without replacing the head unit, I did a fair amount of web trowling last summer. I first identified that the head unit in my car is a Alpine CM5903L, and decided to do the common tape deck modification that directly solders connections from a 3.5mm audio cable to the pins on the tape board. This method requires the insertion of a "blank" tape into the radio to trigger it, and is well covered on other forums so I won't bore you with it more here. For reference, here's what a CM5903L looks like:
I experienced a few issues with this solution, from relatively poor audio quality, static, random pops and hisses, to most annoyingly the tape deck cycling back and forth between tapes 1 & 2. Some days it would be fine and never do it, others it would constantly flip every 5 seconds no matter how many times I ejected and replaced the tape, jiggled the tape, changed the phone and radio volume, etc. I dealt with it through the rest of the summer and since the car stays behind when I'm at school, it didn't get used enough in the last few months to get any more attention on this.
But recently seeing that I have a bit more time on my hands and I am now living at home for the foreseeable future, I set out on another research adventure, finally stumbling onto this thread after a lot of research on the viability of using the CD changer inputs to effectively do the same task as the tape deck mod. Link: http://forums.bimmerforums.com/forum/showthread.php?2018366-DIY-for-Aux-Input-(not-using-the-tape-deck)&p=26647541#post26647541
He covered a lot of the basics such as the connector pinout and the general tech stuff in his thread, so I won't go as in depth here. The general idea is that by using a "jumper" wiring harness that goes between the CD changer plug on the back of the radio and the plug coming from the changer itself, it is possible to replace the audio inputs from the CD changer with inputs from a regular old 3.5mm jack. This tricks the radio into thinking the CD changer is playing (something that prevents you from simply wiring the three audio wires up alone with no connection to the changer whatsoever), while the audio signal is replaced with the music of your choice.
Plug from CD changer that goes into back of radio:
Corresponding receptacle in radio:
CD changer pin diagram into back of radio: (the Sound-System receptacle was absent on this radio, not sure what it was for or which models got it, if any)
Wiring colors used for each pin. Posted mostly for personal reference later.
Note that you don't have to connect the audio L & R negative wires to the 3.5mm plug; you can just use the ground. I remember reading somewhere that those two wires and the ground are all common. I did not verify this, so I cannot confirm this. I used the ground pin in the radio for my audio ground with success.
Here's the part that I felt wasn't very well documented: the physical hardware needed to create the jumper harness. In the quoted write-up, the author mentioned that using a 2x5 computer ribbon cable would work but did not specify where to get one or what to do about the male and female connections needed. I was unable to locate any suitable pre-made cables, so I resorted to making one myself. I started by measuring the pin spacing; this measured to 2.5mm from pin to pin, which I took to be the very common 2.54mm pin spacing standard. After yet more research I decided on using "DuPont" style connectors; they're not my first choice but were cheap and effective.
I then purchased a connector set, a ratcheting crimper, and some decent 24GA wire. Links:
Figuring out what I needed took more time than anything else, and hopefully this saves someone else the trouble.
Learning how to use the crimper was a bit of a learning curve, but well documented and not too bad to get the hang of. I made a big oopsie and completely forgot to get a picture of the finished harness, but the general idea of the harness and what wires g where is covered in the quoted thread. Here's a pic of the harness in progress:
To install, the only other thing that needs to happen is the removal of the CD changer plug from the main connector that goes into the back of the radio for the speaker outputs and such. There's a small blue retainer that slides out like so when pulled on. Remove it, and the CD changer plug comes right out.
Double check plug orientation and install the harness. Note that the little metal tab in the plug from the changer goes down and is a good reference for correct orientation.
I then went out to the garage last night and installed it, and am pleased to report that it worked perfectly!
A bit of intermittent static and popping had me worried at first, but after investigating I'm 90% sure it's my phone causing the problem, not the harness (changing source eliminated the issue; I will follow up with this and update to corfirm the root of the issue). Audio quality is on par with what I was getting out of the CD changer with real CD's, so a definite step up from FM radio or the tape deck mod. Still not amazing, but that's a general e34 problem and not something I care to tackle right now. I routed the 3.5mm cable through the empty spot below the ASC button in front of the shifter, and will be finishing the project tonight by drilling and installing a female 3.5mm jack in the blank plate so that the cable to my phone is removable and the harness won't get ripped out if someone yanks on it.
Good riddance. Will also promptly update with requisite trashcan toss photo sequence, didn't get to it last night.
I do have pictures beyond what I posted here, so if you want anything more or any clarification I'd be happy to provide information. Hopefully this saves someone else the time and frustration of figuring this stuff out! Big shoutout as well to the gentleman that originally posted the writeup (Moron95M3 on bimmerforums), it was a huge help and contained most of the information that I used.