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)...