I expect BFeng will be chiming in soon with an alternative option. John?
Any of the good bluetooth speakers can be used in the car successfully, depending on what your expectations. My 9 year old nephew uses a very small bluetooth speaker in the backseat and it's more than enough for us to 'enjoy' his music from the front seat. Has anyone tried the Jawbone as a handfree phone interface in the car? If yes, I think that'd be a great feature to have.
Headphones in a car ... I tried some ER4's in a car and there was way too much isolation to be safe. I'd say the same is true for well performing ANC headsets, unless they have a mode that allows a reasonable amount of external noise come through (like those from my employer). The sticky point is how much of that ambient should be passed thru to the listener.
Of course, there is a philosophical argument that ANR headsets, if properly designed, could actually improve safety. I have 2 old cars that are loud enough, even at idle, to drown out everthing from a cellphone ringing to an approaching emergency vehicle (until it's pretty close). Any reduction in the sound of the car that improves detection margin for those warning sounds is a good thing. But what if you used the noise reduction to listen to music? Then the benefit wrt safety isn't so clear anymore. But it's not that different than cranking it up in your Audi A8 (which is a really quiet car to begin with). And, in practice the cop who's pulled you over probably isn't interested in this debate.
So, the safe approach is to install a discrete audio system or use a portable speaker.
My personal solutions range from noise reduction ear plugs (Etymotic ER20's or Alpine's ear plugs) to mobile speakers, to a set of ANR earbuds that pass a preset amount of speech-range energy but provide a large reduction outside of that range.