Doesn't your car have Bluetooth? Or rather: how old does a car have to be before it doesn't come with Bluetooth as standard? Every rental I've had in the last five years has had Bluetooth to my recollection, and I don't hire expensive cars.
As pointed out elsewhere in the thread, bluetooth is not a good solution when you want to use one speaker (the car) with several devices. The beauty of a cable is that it's unambiguously connected to whatever it's plugged in to, and not to anything else.
I recently had the joy of having to delete a paired device in a car before I could add a new one. For some reason you could only have three devices paired.
