You can ship the radio back to Harman/Becker (the original OEM manufacturer) and have them add the cable for auxiliary input. I believe they open the case and solder the cable internally, not just plug a cable into a connector on the back.


If you want to retain the original radio, I don't think you can find a more "correct" or factory-ish way to do it.
I found the Becker store on ebay, here's an OEM Crossfire radio they sell with the cable already attached.

http://stores.ebay.com/beckerautosound

http://www.ebay.com/itm/Chrysler-Crossfire-Becker-Infinity-OEM-radio-with-AUX-input-added-BE6806-/263205394762

I sent them an email, asking them if they could add one into my Crossfire radio, and I don't have the email in front of me, but I think they said it would be about $100 if I recall correctly.
Not sure if any shipping would be included.
I haven't done it yet, still trying to find the money to get it done.
Send them an email, and they should answer quickly, and post any additional information you find out.
Their email should be: Service@beckerautosound.com


Checked my emails, 3 months ago they told me this:
Parts Department<parts@beckerautosound.com>
$100.00 plus S/H to add AUX to radio