Yes the Stereo is an amazing piece of kit for the money. Best bit is I just get in the car change the stereo source to BT Audio; press play and it just starts playing the music off my phone. All the car stereo controls ffwd/skip/pause etc.. work the phone while it's in my pocket
Can't think of anything that fits exactly what you after. Nearest thing is a Sony add on that allows you to add AD2P bluetooth to a phone/mp3 player without bluetooth but this still needs a Bluetooth head unit in the car.
Guess you could do what i used to do before i had a bluetooth head unit and just connect the two together with a 3.5mm to 3.5mm lead or even one of the 3.5mm jack to cassette adaptors.
Brodit in-car holders and pro-clip mounts are an excellent option if you plan to use your phone as an MP3 player for the car.
http://www.dsldevelopments.com/