We recently bought a used Toyota Prius that has Bluetooth installed with the JBL audio system. Whenever my phone (stock Nexus 4 running 4.4.4) connects to it, the car sends a "play" command that automatically starts the most recently used music app (Google Play Music, Pandora, etc.). It doesn't matter if no music app was open on the phone, the command opens it up and it starts playing no matter what, and there's no way on the car to stop it from sending the command, as mentioned here. Is there a way to stop this from happening?
There's a Bluetooth connect & Play app that can intercept the incoming "play" command. While it was originally designed to let you start a specific music app playing when a specific Bluetooth device connects, you can instead select "STOP music from playing" to keep any app from starting when the car connects.
Another work around if other method fail, if the android phone always use the same music player to autoplay when bluetooth is connected. Just create a silent mp3 file and rename it AAA.mp3, this will ensure it will be the top of the file list. make it like 5 minutes or 1 hour, whatever you want. file size will be bigger though.
In my case, it will always use the default music player to autoplay. So i just put only that silent mp3 in the playlist. It will still autoplay, but a silent audio. Then I just turn on my favourite music player or podcast.