It may be outdated, but most cars still have tape decks in them. And people still have a soft spot for the cassette for some reason. combine the two and you have the Flexii Bluetooth Cassette Adapter, which can be used to connect wirelessly to a mobile headset, media player, or even a PC.
Tape adapters are the way to go, since they’re easy and cheap and work pretty flawlessly. It has an internal lithium-ion battery good for about 5 hours, car charger, and a USB cable. It will cost for $46 shipped.