It's one cable from the bottom of the mdi unit to the back of the RNS, there's nothing else in between it. You will have to remove the RNS to fit the cable.
I think what Darren was referring to is that your existing Ipod dock isn't capable of any track name or control over it no matter what cable you use. The good thing is though that if you already have the armrest ipod dock, then it's a doddle to fit the MDI dock in place if it and plug the new cable in.
So basically you need 3 parts. An MDI dock unit, the cable and the ipod adaptor cable that plugs into the mdi unit and then plugs into the bottom of the ipod. There's also a USB cable version that would allow you to use USB memory sticks instead of an ipod.