I use an
Acoustic Research ARWH2 2.4 GHz Mini-Bridge Stereo Headset in my car. These can be found for around $20. I connect the ARWH2 to my car stereo (using a 1/8'' male-to-male plug) an dcharge it via USB. It syncs easily with my cellphone and allows me to play my music via Bluetooth (in high-quality stereo) without having to upgrade to a bluetooth-enabled head unit. It does help that the aux input for my stereo and a power source are in my car's center console (under the armrest).