I will eventually make a tutorial, but the main line is this:
a) locate the cables coming from the BT unit (under the seat) to the plug that goes into the stock HU
b) there should be 3 cables, (+, ground and interrupt signal), extract them from the plug
c) take positive and ground and install an RCA plug
d) both this and the interrupt cable go into the KCE-900E, which in turn is connected to the Alpine HU.
e) set the Alpine HU Aux input (one of them anyway) into "Phone" etc
Presto, each time you use the phone, audio is interrupted and the voice is heard through the speakers. Basically all functions as before. All you have to do is adjust the volume (having said that, it is a bit annoying that the "connect" and "disconnect" sounds are much louder than the conversation volume).
Hope this helps.
Nick
PS: I know my installer also used a relay at some point, I'm not sure where that went though, I'll have to ask and get back to you.