I recently got a Bluetooth FM transmitter for $23.99. Connects your phone to the car radio via an open radio channel and will play music and transmit calls. Also has spots for a micro SD and USB if you want to load your music that way. Really handy if you have not so modern modern vehicles (our collection ranges from 1957 to 2012). Got one for my husband too.

Everyone is saying that a Bluetooth cigarette lighter thing doesn't work great and maybe an actual car radio with Bluetooth would be better but I have one and I'm happy with it. Cigarette lighter Bluetooth thing on Amazon