The Ibus wire is most easily available on the 3 wire connector in the trunk that is for the phone module or maybe the cd changer module (i forget which) -- It's in that big taped up bundle. It's small, so it's hard to find, but trust me -- it's in there! The trick is that the color of the wires are not consistent between models of car, well except that the brown wire is GND.
On the 3 wire connector there are the same wires you need: gnd, 12v, and ibus.. You can find and verify GND pretty easy with a voltmeter, but Ibus and 12V look the same to a voltmeter. The best way to find Ibus (the way i did it) is to use an oscilloscope between GND and each of the two wires until you find the one that has data on it.. Ibus will be 12v, but you will see it drop to 0V when it sends a bit. (Inverted signalling logic 12V = 0, 0V = 1) Turn the key to position 1 and leave the door open so that your car 'dings' -- the ding is triggered over the ibus, so this will ensure some traffic on the bus. Then just start probing. You will find it right away with the scope.
If you hook it up backwards, you'll likely lose the 12v line because you will have blown the telephone fuse.