Just to chime in: I have an iPhone 5 and a Mitsubishi ASX. Everything worked fine, before iOS 8, I would connect my phone to the USB port, and then control everything from the steering wheel: next and previous track, volume, etc. I could even select playlists or specific songs and all that.
Now... it behaves erratically: sometimes it reports only 1 track or 4, or 6 (I have around 200 on my phone, right now). It rarely starts playing by itself, I need to go into the music app and hit play. The track skipping doesn't work, it just changes the numbers on the car stereo screen, but the tracks don't actually change on my phone. It's a huge mess!
If I reboot my phone, it works! everything works after a reboot. But the next day, it's all back to being a huge mess... until I reboot again.
Pedro