If you drive an older car or truck, then chances are that you cannot hear music or audio streams playing on your smartphone through your car audio system. One simple solution I use is the CCrane Digital FM Transmitter 2.
The CCrane FM Transmitter 2 comes with a Volume dial that allows you to adjust how much sound you are pushing through the unit. The goal is to push make the signal as loud as possible, before the included LED light goes from green to red. If it is red, you are putting through too loud a sound and the resulting playback will sound distorted through your car audio system.
The unit also has an extendable FM antenna, to improve the signal output. (You can use the FM Transmitter 2 to broadcast audio to regular FM radios throughout your entire house, in fact.) I have used this unit for years: It is a great way to stream web audio through my 10 year-old van’s audio system. US$49.99 at www.ccrane.com/transmitter.