Once Apple's iPod took off, many car companies instantly realized the potential of the small device with regards to how people listen to music in their vehicles. Companies like BMW, Kia, Hyundai and Nissan offered iPod docks in their cars that can very easily work with the device.
These docks utilize the "30-pin" connector, which is the slot on the bottom of modern iPods and iPhones, according to CNET. This means the driver needs no cable, they can simply slide the iPhone or iPod directly onto the receiver. However, Apple recently revealed it is considering moving to a "19-pin" connector for the upcoming iPhone 5.
For the companies that chose to put a 30-pin connector in their cars, the new iPhone 5 wouldn't be compatible. The music player would also lose compatibility with many of the home docking stations built for the device by third parties in the past.
This wouldn't necessarily be the end of the world for Apple fans, however. Many cars instead use a USB connector for music streaming, meaning the driver could just use the cable included with iPhones/iPods to hook up their device (assuming Apple sticks with USB). In addition, it wouldn't be surprising to see Apple or a third party release an adapter for the 19-pin slot.