Amjimshop New USB Bluetooth 3.5mm Audio Stereo Music Receiver for Iphone 4 5 Car Mp3 Black
Does exactly what is advertised. The only small drawback is that I need to connect the phone to it every time I get in the car (I just select the adapter from the list and it connects, so it might have to do with my phone...), but apart from that it's great. And it's so small that it can be in a double USB adapter next to my phone charging cable, which another model I tried couldn't (too thick).