This worked great for me. My car (Chevy Cruze) supports bluetooth, but not the A2DP required for streaming music.
I purchased this:
Amazon.com: Miccus BluBridge Mini-Jack RX: Bluetooth Music Receiver use with Bluetooth-Enabled Mobile Phones, Tablets, PCs and Apple (BBMR-02): MP3 Players & Accessories
You plug this into your AUX input, and if you have a USB port nearby you can plug that into the receiver to keep it powered.
The Nexus will bluetooth stream (A2DP) to this box, and the box plays it through the AUX input and out your stereo speakers.
Pandora, GMusic, Notifications, Navigation, everything comes across my car's speakers now. When I get a call, the music pauses, I take the call, when I hang up everything resumes.
Great device for cars that don't have A2DP streaming ability, and for the MTP situation we have. I couldn't control my phone via USB either, but I can my iPod. I'd rather use the Nexus, and this makes it all work.
Hope that helps.