I also added bluetooth capability a year ago and am very happy with it.
Its less than $100.
I started with an iSimple Aux Input unit (I do not have the CD remote, and did not want to dig into the radio head unit) to add stereo input capability to the Bose headunit. It comes with a wire harness which simply plugs into the back of the radio and provides a small box with a stereo input cable. You can plug the cable directly into your phone to see how you like Pandora played through the Bose system.
I liked it...so I added bluetooth capability.
I decided to use bluetooth only for music. I ordered a HomeSpot NFC-Enabled Bluetooth Audio Receiver and a very small ( one x 1/2 inch) potted 12 V to 5V converter from the auction site (under $8 direct from China). It had 12 V wires on one side and a six inch USB female connector pigtail on the other end. I wired the 12V side to the accessory power connector. The USB 5V side powers the Homespot.
The ISimple box, 5V converter, and Homespot were all placed under the radio compartment and are not visible. The Homespot automatically connects to the phone when I use the phone bluetooth setting.
I liked it so much I hooked another Homespot to my home stereo system.
To hold my cellphone, I use a Satechi Universal Smartphone CD Slot Mount for 3.5" - 5.5" Smartphones. It mounts into the CD slot (do not use CDs anymore now that I have Pandora). It places the cellphone in an optimum position and is very sturdy. It is really nice when using Pandora or GPS / Waze on the phone.
I now have the two features of importance to me in my C5: Bluetooth Pandora and GPS / Waze.
One last item is that I replaced the smaller speakers in the doors.