For starters, this is NOT an original design, the VWCDPIC is someone else's creation, which you can read all about over at K9spud .  This project is based on the VWCDPIC 3.x design, it's just my own PCB layout, which I did to adjust it to what I wanted.  I removed the on-board programming functionality, and the DB25 expansion connector, placing two PCB-mount RCA jacks for audio input instead.  I also added the external power switching functionality found in the "Power control board" schematic on their site to my main board, as a precaution against some of my suspicions about the resetting issues I had in my original board. 

 What the VWCDPIC is:  (from the designer)
"The VWCDPIC is a "lock pick," if you will, that unlocks the CD Changer interface by sending the required data stream, fooling the head unit into thinking a CD Changer is connected. With a VWCDPIC, you'll be able to listen to any audio player through your stock OEM head unit. You can often remotely control most MP3 players from the head unit's buttons as well (PJRC, WinAmp, Archos Jukebox, iPod, Pocket PC PDA, etc)."

My original VWCDPIC had some issues - it would reset if the volume were turned up too loud.  It also suffered a burned-out trace on the PCB.  And, it would occasionally work itself loose because of the homemade connector, usually losing left or right channel audio, requiring me to go to the trunk and mess with it to get it going again.  And, recently it just failed on me entirely.  I never really solved any of the problems, I just decided it was time to give up and build a new one, doing a better job this time and hoping for the best.

 The PCB was designed to fit into this cheap project box that I had around.  The board is mounted with standoffs to the bottom of the case, and the lid is cut to make room for the connectors and LED indicator.

 The device connects to the VW CD Changer connector in my trunk.  This time, instead of a homemade connector I used an actual VW connector, which I purchased (with the pre-wired pins) from World Impex using the part numbers found at K9spud.   This ended up costing me something like $20 total, which is steep for a single connector, but I think it's worth it considering what a hassle the homemade connector was...

The first time I plugged this in, I heard a muffled POP and quickly unplugged it.  Turns out I had accidentally used a 2N3904 NPN transistor in place of the 2N7000 NMOS, and 12v applied to the base pretty quickly obliterated it.  After replacing it with the right transistor it works fine.

Unfortunately, the head unit is still resetting on me when the volume is too high.  It's not as bad as it was before, but it's still annoying.  Now that I've tried several adapters which others have had success with, I'm pretty confident that the problem must be elsewhere.  Either my head unit has a problem (it's sort of messed up in other ways anyway) or somehow the ground loop isolator I'm driving the audio signal through into the adapter is causing some issues.  But in any case, at least it's usable.