The system from factory isn't bad. I'm driving around all the day and it works for listening to the radio or cd...

but I have another question. There are 4 speakers in the GT3. 2 in the front 2 behinde the seats. But in the GT3 I drive, only the front speakers are connected. I thougt that the rear-speakers are not built in. when I turn the volume with the fader in the beckerradio, from front to rear, the sound vanishes. - so I think the rear-speakers are not speakers, there are just faceplates? is that true? or is something wrong with the cables, connection?