The easiest place to find the iBus is on the CD Changer connection. It's a simple multi point bus, so you can 'T' it off if you want to keep the CD Changer (or other device connected to it).

The CD Changer has two connections, and audio connection which is either a 6 pin plug, or a co-axial blug if you have digital DSP, and a three pin iBus plug with +ve, Gnd and iBus data wires.