Do you have a Bluetioth capable audio system ? Some older vehicle systems have bluetooth handsfree but not audio. Two Bluetooth devices need to be "paired" to each other. All of my vehicles (Toyotas) allow me to pair my phone and the sounds system as either handsfree, audio playback or both. From memory that question is asked when initially pairing and sometimes you need to un-pair and re-pair to get it right (just once). Your vehicle's manual is the place to start and iPhone prodcedures are well documented online.

There hundreds of bluetooth speakers available now, from the smallest to the largest. Just pair them up with your phone and whatever you choose to play comes through the speaker, music,radio or whatever.
They just charge up with a usb cable and are fully portable, some waterproof for boating, others with a lanyard to hang from the awning or good for music around a campfire, endless possibilities.

Cheers Bob