The initial convo went:
> Paying with your phone just seems like one of those separation of concerns problem.
Followed by:
> You could then separate the audio system from your car and drive around with a boombox
The first discusses behaviors of end-users. The second was a lame attempt to take the mickey of that, which is why my response clearly indicated that, to my knowledge there are zero security implications of playing your music through your car's entertainment system.
This remains true.
You are discussing design flaws, not user behavior flaws, which is why I pointed out that the design flaws you bring up, in addition to doing you bodily harm, could conceivably also be part of an exploit chain that validates the original poster's concern about using his phone for banking.
But I still sincerely doubt that the choice of playing Beethoven or Iron Maiden either directly places you at risk*, or makes a difference to the ease of exploiting any design flaws in your vehicle.
IOW, the first behavior given (not using your phone for banking) is easy to construe as prophylactic, given that, yes, in fact, peoples' credentials have been stolen from their phones and bad things have happened, due to using phones for financial transactions.
The second behavior given (use a boombox instead of your car's audio) of course could theoretically alter outcomes, but to my knowledge, there has never been a car exploit that depends on whether you have fiddled with the volume control or station selector.
* Assuming of course, that your volume isn't so loud that you've riled up other people. That's always a risk.