If you don't want to spend money on an integrated Bluetooth stereo for your car, repurposing blog Scrapyard Electronic shows us a way to convert an old pair of Bluetooth headphones into a receiver. The end result is a Bluetooth stereo system that costs next to nothing.
The process of hooking the headphones into the stereo is pretty simple, but powering them is another story. Scrapyard Electronic pulled out the components from a pair of Nokia Bluetooth headphones and ran power to them through the car's battery. It takes a bit of work to get it going. If you want to try, you can find the full process over on Scrapyard Electronic.