I would say the best route would be to go from the center console under the drivers seat, then down the side of the trim on the floor under the threshold of the door. After that go behind the rear seat and into the trunk. I did this in my S class 140. You could also pull the carpet up and run it right underneath. I thought the Denison kit hooked into the head unit? I have not seen one of those and for your model it would be fiber so I don't know. My car is hard wired and I just unplugged my CD changer from the head unit, pulugged in the a NEO and ran the cable down the center to my console, iPod plugs in there and I can control from the head unit. I am looking at a iCruz from Monster too. Check out these sites although I think Denison is the only one making one for fiber. The Ipod hooked to the Bose system in my SL is pure heaven and when I got the Sirius integrated that was a pleasure as well. Good luck!
Brian
http://www.mp3yourcar.com/
http://www.alpine-usa.com/ipod/
This one is if you decide to replace your head unit.
http://www.monstercable.com/icruze/