I use the following in my 2 yr old santro xing erlx
1) Sony cdx-gt-350s with front aux-in
2) Sony front speakers and sony 4 way rear speakers
3) Sony amplifier in 2.1 mode i.e. rear speakers + sub
4) Fusion 12" sub tube - no more space in my boot
5) Archos 504 30gb - my fave accessory - plays audio/video to my car receiver (or tv or onkyo thx), records audio/video from any source, and is a usb hard disk. upgraded to this from an archos 100gb av500 with similar functionality.
Archos & front-aux-in are the best things that has happened to me
when i do not have charge in my archos, i can even play music of my phone or laptop using front-aux-in. Also have a cigarette lighter power strip (2 points), using which I can charge ANY 2 devices (helps on long journeys - mostly charge the laptop from lighter and charge phone and archos via usb)
Cheers,
j