I believe (standard disclaimer that I may be incorrect) that the Plus 4 SS stereo has been kept VERY simple. It is my understanding that the car has speakers and a head unit which allows you to connect an MP3/iPod to it. You control the music via the MP3/iPod and the unit simply amplifies the sound, so that you get enough grunt through the speakers to hear the music. It is very simplistic but effective; it also means the weight is kept to a minimum. It is my understanding that you can also delete the unit if you so choose.
As I said, this is my understanding, based on a chat with the chaps at Aero Racing quite a few months back.
