You can have the existing factory radio modified by Becker (the original manufacturer), to provide an auxiliary input.
I sent my factory radio to them, and they added an internal Bluetooth interfaceinside the unit.
So when I enter the car, my phone automatically connects to the radio, and I can stream music from my phone, like from my Sirius satellite subscription directly into the radio.
The Bluetooth is only for music, you can't use the phone through the radio for calls, just music.
Cost for this factory modification was $148. including paying their shipping.
www.beckerautosound.com
They have a phone number listed there, call them and ask if you have more questions. It's cheapest if you send them your working radio, otherwise they sell modified radios on eBay for a higher price.

They will also alternately add an audio input cable with jack on the end of the cable. You can mount the jack (like inside the armrest) if you'd rather use a cable.