Having finally decided to ditch my old car after 8 years and buy an xfire SRT, i was interested in this thread. For the past 8 years I have used a portable mini-disk player (powered from the car via a 12 to 3V adaptor) and this gave great sound through my old car stereo. I used a tape head adaptor as my old car stereo was radio/cassette (remember those?). So now my xfire does not have cassette, I can't see a way to connect the mini-disk player to the car sound system.
I would want a system where the radio volume/bass/treble also controls the mini-disk player output, and the adaptors mentioned here, bypass the head unit, so obviously there are no such radio controls available. I also noticed the SRT may have "optical in", but as the mini-disk player has no optical out (in only), this is also a non-starter.
Even if I hacked into the head unit (hacked maybe the wrong word) to find a suitable input on the CD circuit, the radio and amp would need to be powered up, usually done by the CD insert mechanism plus power switch. So again, not really an option
In conclusion, I think I will opt for a new head unit with aux input and even then I have to consider how to connect to the 6 speaker power amp <sigh>
Maybe I will just burn CDs again. I dont suppose the 06 SRT player can hadle DVD audio? I wish!

Dave