I don't think very many of us (or maybe anyone) can say who is at "fault" here. The iPhone 5 works fine with many cars and the iPhone 4 works fine with the cars with which the iPhone 5 has problems. Maybe the iPhone 5 does not properly support the BT protocol; maybe the cars don't. Maybe they both do, but the protocol leaves too much room in specs that two devices might not communicate properly. I know I have no idea. All I know is that I've got a very expensive car and very expensive phone (relatively speaking) that don't work properly togehter.
I really don't think it helps for either vendor to merely state that "we haven't tested for compatilibity" and that if we do, and we verify what you are all saying, that we just won't list it as compatible, and screw you early adopters. The problem affects a wide enough range of cars and head units that if Apple fails to fix this, it's safe to say they will lose customers going forward. Similarly, the iPhone 5 is a hugely popular phone, and once this become widely know, whowould buy a car that was not compatible with it? Both sides are going to lose significant business if this situation is allowed to continue for very long.
In the end, though, I think it comes down to this. A car does many things. Communication with a phone is a small part of that, and is not within the expertise of the car companies. I would say that a very large percentage of car companies merely buy the BT as part of the audio head unit. They don't manufacter it. BT, however is a very large part of an iPhone. Apple has great expertise in creating software for their phone and other computers. While I'm sure the head unit manufacturers are looking into this (they do have the expertise, to roll out an update via the various car manufacturers that is compatible not only with each car, but with a method of installation specific to that car, is going to be difficult for all concerned, including us.
However, for Apple to roll out a fix, OTA, is a much simpler matter for everyone. Apple, however, has to insure compatibility with all of the head units with this problem and not break compatibility with other BT devices. Also, Apple is not likely to roll out an update for this single problem. I'm sure there are a host of problems they are dealing with, and that the first update will fix multiple issues (problems with battery life and with the phone preferring phone network over Wifi, and uploading tons of data are merely two well reported ones). With luck we'll see something soon. Hopefully this is not a hardware problem that cannot be fixed in software, since the one thing we do know is that that the BT chipset in the 5 comes from a different manufacturer than the chipset in earlier phones.
Everyone with the problem, however, needs to get on the phone to talk to an Appletech no matter how fruitless it may seem. Every call is logged, and hopefully every car type or headunit type will be logged too. The more calls, and the more incompatible devices, the more likely Apple will deal with in promptly.