GM/Delphi/Delco Protocol Hacking

Hi all. I'm doing a bit of hacking with my car's audio system, and I have a few questions.
The short version is: I'm looking for all information related to
GM's/Delco's use of the Class 2 data bus to control CD Changer-Head Unit interaction.
The long version goes something like this...
I drive a 2004 GMC Canyon, and I'm interested in doing some fun things with the data that gets passed around the communications bus. My first goal is to hook a small computer to the CD Changer interface of the radio. With this, I'd like to be able to play MP3s in my car, while displaying ID3 tag info on the head unit, and controlling the playlist with the head unit's buttons.
Now, before too many people get started with alternate solutions, let me say that the purpose of this project is not necessarily just to be able to play MP3s in my car, but rather to have fun hacking the serial bus and doing an interesting project. So, I'm not into iPods or pre-boxed auxiliary adapters.
Here's the data I have so far, in hopes to lure others who are interested in similar projects to colaborate with me:
To start with, the head unit in my car is a Delphi Delco Electronics, part no. 15104155. Just Googling for that part number brings up the site: rod_detail.asp?pn359576 which is a supposedly backward compatible head unit (according to the spec sheet they publish on that site).
I contacted the dealer at the start of my project, and was able to get the pinouts for both the main head unit-car connect, and the head unit-cd changer connection. If people want those, I'd be happy to post them here - the dealer didn't say anything about them being copyright restricted or anything. The important parts are that it is a 12-pin connector, and the only pin in use that is not labeled as an audio signal is labeled as "Class 2 Serial Data".
So, I searched around the 'net for a while and finally figured out that "Class 2" is GM's implementation of SAE's J1850, in VPW mode, with part of SAE's J2178. I have obtained copies of each of these specs, but they are proprietary, and not given away for free, so I'm afraid I won't be able to post them here. In any case, they don't list anything about audio device controls, other than the suggested range of addresses for such devices.
Once I knew which kind of bus they used, I bought myself a VIA LDV100P1 from B&B Electronics. This little module can convert data from J1850-VPW to RS-232 and back. So, I've written a small terminal program, and I've been snooping my car's bus for interesting data.
What I was hoping to find was that the CD player, even though built into the head unit, was passing information back and forth to the head unit's display over the Class 2 bus. No such luck, unfortunately. In fact, the only data I have seen sent to the audio range of addresses is targetted at the Audible Warnings devices.
My next thoughts center around buying a compatible CD changer to install, and then snooping the communication on the bus (as data would absolutely have to be there for an external module). The only problem with this strategy is that I have been unable to find a compatible CD changer. The closest I've come is USASpec's SCD-G7i, but they say that there is a software incompatibility with the 2004 Canyon. I find this a bit suspect, as the G7i is supposed to work with the 2005 Canyon, as well as the '04 and '05 Colorado, and the head unit I talked about Googling for above says it's compatible with all 4 models. But, it's hard to say that the company that developed the instrument is wrong about it's capabilities, you know? ;) Also, I think I remember reading somewhere that the 2005 Canyon actually uses a Bose system, not a Delco.
As any of you have tried will know, my phone calls and emails to places that should know this information have turned up absolutely nothing. No one at GM seems to have access to the data I want. Delphi has given me two replies: 1. GM should have that data, and 2. their engineers don't have enough time to reply to my request. Helm, Inc., GM's official literature distributor, claims to have nothing that details CD changer communication. The Chilton's manuals don't even touch audio equipment. My last resort was contacting PIE and PAC to find out how they got the documentation they needed to create their auxiliary adapter modules. They both replied that they had to reverse engineer the communication themselves, and could not release the data that they gathered.
So, that's most of what I know for now. Any pointers for other people to contact about this issue, or other websites to read would be great. And, if I've left out important details, please ask - my data is a bit spread out and sometimes hard to compile, but it's probably here.
Thanks, Bryan
(P.S. My appologies for the repost to those who are also on I had this over there for a few days, but didn't get a response, so I thought I would probe interest in this group.)
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