stock stereo and engine diagnostics

I have a 2000 jetta TDI, and recently replaced the stock stereo with an Alpine CD player. A number of months after the upgrade the check engine light came on, and when I took it to the dealer, they said they could not get a reading with their computer because I did not have the stock stereo in the car. I went to the stereo shop where I had the cd player installed, and they had never heard of anything like that before but replaced the stock deck anyway. I then went back to the dealer and they were able to determine the problem. It seems strange that the solution to the problem is swapping the stock stereo in and out to test the electronic diagnostics. Is there some other solution that anyone knows about?

uh. that's stupid. I have an aftermarket deck, and I've had my check engine light checked several times by AUTOZONE and they didn't have any problems finding out what it was, and they will do it for free. it seems to me that dealers are really the LAST place people should be going...and only if they want a runaround.

Here's the deal. On the OBDII volkswagen up to 2001 or 2002 there is a wire called the k-wire. This is used on the vag-com to get readings from the stock stereo i.e., if a disc is in, if it's jammed, if things are working fine. A lot of aftermarket wiring harnesses connect this k-line to a +12V line which can damage a vag tool. The 2003s and possibly 2002s have a com+ and com- line used in a similar fashion to the k-wire.

Here's the location of the k-wire:

http://dave.koston.org/pictures/radio-k.gif

Here's the deal. On the OBDII volkswagen up to 2001 or 2002 there is a wire called the k-wire. This is used on the vag-com to get readings from the stock stereo i.e., if a disc is in, if it's jammed, if things are working fine. A lot of aftermarket wiring harnesses connect this k-line to a +12V line which can damage a vag tool. The 2003s and possibly 2002s have a com+ and com- line used in a similar fashion to the k-wire.

Here's the location of the k-wire:

http://dave.koston.org/pictures/radio-k.gif

Thanks for the information. The dealer said something about a wire in the harness that needed to be looped but they did not say which one, and the guys at the stereo shop did not know what this was all about, so now I have some information that I can work with.
Thanks

Do any of you know how the functions of each of the wires in the wiring harnesses ?

I want to add a third audio source. There is an empty input from the changer (which I don't have) but can't find wiring descriptions. Do any of your manuals have this detailed description? I want to put audio into this input and find out how it know the changer is present.

HELP... any ideas? E-Mail me direct. Michael