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Re: Pulled out my 2002 dbl-DIN CD, pics & wiring diagram (UKAUSSI)

Thanks for the informative pix and info above...
One small bit of insight... The CAN bus is a form of internal software communication used within the Auto Industry. Two reasonable GUESSES, based on what I know about CAN, are:
1. The HU does, indeed, need to be connected for the analytical equipment used by VW to work. Analogous to IEEE-488 or RS-485 communication loops, the HU represents a "node" or piece of equipment on the CAN bus, and test equipment needs to be instructed that it is not there; perhaps not a test mode that stock VW test gear has enabled. Likewise, when you plug and unplug a piece of USB bus equipment, it has to go through the "discovery" mode where it is recognized on your Computer.
2. CAN is very different from the K wire protocol. Whatever applied about having the K wire connected or not connected during test modes likely does not have anything to do with the set of questions above re: the K wire.
I don't deal with CAN bus issues every day like my work pals, so I can't say just yet on how one avoids having to connect the HU back in during Service visits.
It's much easier on Car Manufacturers to send software commands via the CAN bus to dim HU illumination lights, for example, than to run a separate Illumination wire. So, the CAN bus allows much more Command and Control activity on a wire pair w/o running wires for each little Auto function. This saves money and wire harness weight. Sounds silly but, yes, a lot of little incremental savings is how Manufacturers cut down on cost and weight, not thinking of how these designs affects things like Aftermarket sound system flexibility. Hopefully, some VAG wizard will tell us all what the changes in the test mode commands are, and you can then inform the VW Service Techs on what to do to get around these test roadblocks.
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