I know I'm a little late to the party, but I just recently got a Bluetooth-enabled radio installed in my older car. I've never had such before, and I LOVE it. You can get them at Best Buy, and have them installed there for around $70 if you don't want to do it yourself.
I also love Spotify. The "pro" plan is cheap, especially if you do a family plan which they recently started offering.
Here's my problem: When my car connects to my Android phone, it starts to play the default Android mp3 player-- and nothing I could do (at the time of this writing) could change that. I found helpful apps online which would *start* Spotify when the car connected, but nothing that would make it start *playing*. I still had to do that manually on my phone.
But then I found a solution (for Android anyway)...