My 155 had one fitted when I bought it, but I've installed CARK91s in two cars before (A Rover and an MX5), and it's very easy.
Not too familar with the 155 electrics (yet!). If it's any help I can have a look at the install and let you know where everything has been put. To get you started, the mic lead runs up the a-post trim and sits above the drivers side window, and the speaker is somewhere in the passenger footwell (sounds like it is anyway). The mic looks like it's too far back, but seems to work fine. Tax disc antenna works well. The 155 install was a professional one.
Might even be able to manage some pics.