Well I don't understand how they code their engines i.e.

S50B30 Euro 3.0
S50B32 Euro 3.2

For the US, some say the engines are

S50US US 3.0
S52US US 3.2
eg: http://www.bentleypublishers.com/product.htm?code=b398

and then some sites refer to

S50US B30 3.0
S50US B32 3.2
eg : http://www.bcw3design.com/m3/us_euro_faq.htm

So I really don't know what's what.

Anyhoo... With regards to your Ibus problem. A volt meter will show nothing of value. I'm not sure about ground being brown, but if your unsure find a bare chassis bolt and temporarily use that as ground.

To see a signal you need an oscilloscope. The Ibus signal is pulled high to +12V so it should read +12V most of the time and very very briefly dip when something is transmitted.

There will be very little traffic if the CD changer is disconnected. The head unit will only transmit about once every 30 seconds looking for the changer.

If you have an Ibus changer, the CD changer should have two connectors. 1 x 6 pin connector for the audio and 1 x 3 pin connector for power, ground and Ibus.

I just built the basic circuit above and connected it up with the CD changer still connected and watched all the packets go backwards and forwards.

Have you seen the HackTheIbus Yahoo group?
http://groups.yahoo.com/group/HackTheIBus/

Specifically I used:
http://groups.yahoo.com/group/HackTheIBus/files/IBUS%20Interfaces/SimpleInterface2.jpg
http://groups.yahoo.com/group/HackTheIBu...deDRAFTREV5.PDF
and
http://groups.yahoo.com/group/HackTheIBu..._Connectors.pdf

You probably have to be a member to get the files.