i've been looking an looking and pretty much all im seeing is the ipod interface modules. is there not a simple plug that taps the audio input pins on the cd changer input on the back of the radio and gives you a 3.5mm headphone style jack on the other side that you can plug a phone, ipad, or anything else into?

 

Pretty sure Blitzsafe offers an rca version that you can get a simple adapter from rca to 3.5mm. But I think even that is more than you want to spend. I haven't seen a real cheap alternative yet.