Hackers can do anything they want I saw today that people are concerned that the voice controll on a mobile phone can hacked if you are using headphones
And someone with a laptop antenna and amplifier can send rogue electromagnetic waves to your earphones that will interpretation will be that you are asking you mobile to do things and they can pretty much do anything with your phone that you can by talking to it
And thinking about it now I used to have a app on my phone like 5 years ago that allowed hidden remote controll of a phone via Bluetooth all you had to do was accept my Bluetooth connection and I had controll of your phone. That was 5 years ago
That app was on a very old lg mobile phone before iPhones I never used it to harm people I showed them the app so they new that people could do that sort of stuff and that allowed camera controll, calls listen to you call, make a call myself and use your credit, sms texts , email, core to copy of your phone contacts and messages to my phone , a lot more that I can't remember and like I say that was 5 years ago.
I imagine if I wanted to do that now wifi would be better than Bluetooth and most people if any that are trying to hack your phone would use a wifi connection
1) there are more people that leave there wifi on and connect to insecure networks allowing for more targets and minimal tools just a laptop and software.
2) wifi is faster than Bluetooth allowing me to copy more of you data in the moment that I have got access to your data
3) wifi has more protocols
Essentially everything can be hacked by any one who has got the tools and the knowledge but it's normally your own stupidity that gets you hacked eg weak passwords and connecting to insecure networks with wifi is a huge risk you don't no who else is on the network and what they are doing
I would say bluetooths pretty safe these days as wifi attacks would be a better approach and I would have be in a 10 meter distance of you at all times or the connection would be lost so I would be pretty much drivng at the side of you all the way to your destination
On the app I had all that time ago no if you didn't accept the connection you couldent do anything.
But There's always a chance someone could do something the same but not need your permission but like I say you say you would be using it for music in your car some one would have the be in your car or driving next to you to maintain a connection timing would be impossible from outside your car because of the short range of Bluetooth
Your main concern would be passengers in your car and how many people do you no that are that skilled with computers to actually hack a device in that way would need vast knowledge of computers espessial If they wanted to do it without your permission or you actually accepting, or letting someone else accept the connection on your phone .
If you worried about people outside your car hacking you I wouldent worry at all
If you are worried about your passenger hacking you while you both travel together then just watch what you accept on your phone n don't give other people access to your phone