If your car do not come with Aux input or Bluetooth connection. FM Transmitter is the only option that allow you to play / stream music from your smartphone thru your car radio.
This is how it works. Connect the best fm transmitter to your phone’s headphone jack / adapter to the transmitter. The transmitter will convert the music signal to radio signal on a specific frequency. Set your car radio receiver to that specific frequency and music will be sound thru the car speakers.
If your car still has cassette and CD player in it. It will be fairly easy to swap out the old one with a new double din stereo which are relatively inexpensive today.
If the car is 2000+, it will not be as easy since most 2000+ car comes with integrated with other electronic devices such as climate control / display / GPS and etc. Messing with electrical in a late model car is not recommended. Since you may disable other features. In the worst case, you may even disable and kill the vehicle.
If you wish to add smartphone features such as streaming music, GPS , and talk handsfree you may want to look for a fm transmitter as alternative.
Gizmo Guy Gadget is a simple wireless gadget that allow you to integrate your new smartphone feature thru your factory radio without the hassle and expense of a new radio. It use proprietary technology to eliminate the humming and static noises that is common in old traditional fm transmitter. It provides 100% crystal clear sound with NO STATIC. It is truly the best fm transmitter for cars without Aux or Bluetooth.