If you have an old car with no bluetooth, there are adapter kits for around $200 that allow you to splice bluetooth directly into the audio system. The audio quality will be much better than the FM transmitter and you won't have to constantly switch frequencies on long trips. Many also come with a microphone for phone calls.

The install is easy, generally requiring only interior pry tools (< $10), screwdriver, and some adhesive/zipties to secure the module inside your dashboard.