FWIW, I'm posting my solution to this problem a couple years after the last contribution to this thread in case someone else (like me) might encounter this problem and search for solutions.
I have a used 2012 JSW TDI that I bought a month ago, as well as an iPhone 4s running iOS8. Like a lot of other people on this forum and others, I noticed my iPhone would automatically start playing music/podcasts/etc over the bluetooth upon starting the car. This would happen regardless of if auto-play was enabled in the car stereo preferences and whether or not the "media" button and bluetooth audio were your current selection. As you can imagine, this would serve to both drain the phone battery, as well as cause you to lose your place in a podcast episode. If auto-play in the menu was selected, it would cause the radio to switch over from, say, FM radio, to the bluetooth audio. It would also cause a switch over to bluetooth if you were, for example, using the phone's GPS and the phone issued a verbal instruction about an upcoming turn. All in all, a pretty undesirable quirk in its programming.
Note: Before I got the problem fixed, I did select bluetooth auto-play in the menu because I'd rather know that the phone's playing and stop it rather than have it play (using battery and perhaps data) unknown to me.
Because my car was still under warranty, and this problem was still a huge pain in the neck and, frankly, an unsafe distraction for this stuff to start happening while operating the car, I took my car in to the dealership for a radio firmware update. Buried in a thread on an Apple forum, someone else suggested this. Getting a firmware update as well as deselecting bluetooth autoplay in the car's stereo menu solved the problem.
Additional note: Having auto-play enabled after the firmware update will cause it to play automatically. But this is actually logical functioning, in this case. If you don't want to it to play automatically, de-select auto-play. In short, the update makes the auto-play function properly.
I can't speak to any other conditions besides having a 2012 TDI and an iPhone 4s that would cause this particular problem, but there did seem to be duplication of the problem across at least a few model years of VW cars and multiple generations of iPhones and iOS iterations. I've also seen mention of the same problem with Android phones. But this is what worked for me, and I hope it stays that way. Seems to have been a problem on VW's end, and not with the programming of the iPhone.
I will say that despite my car being under warranty, the dealership guy was still slightly reluctant to do the update for free. But I made a pretty succinct case for this impacting the proper operation of the car stereo, and didn't have to argue too much. It will apparently take the dealership several hours to download and update the stereo's software, so not exactly an in-and-out operation.
