I don't know what the trunk (boot) of the Roadster is like, but the coupe has two styrofoam filler units in the back. The driver's side one (in the states) has the jack and TyreFit stuff, the passenger side (in the states) has a spot where it was noted a first aid kit went in the UK.

I used this 'first aid kit' compartment by making it deeper and lining it with 3/4" soft foam. I then ran my cables along the passenger side of the vehicle following the factory wiring, which goes down the middle of the passenger side floor, under the seat and then slightly toward the outside of the body and through a factory opening in the 'wall' behind the seat. It continues to the security module in the rear. I then threaded the cable under the styrofoam from there to the CD-Changer.

Hope that helps. Let me know if you need a picture of the changer in place for reference.

Cheers!