The car will natively decode a number of popular formats, including MP3. Just stick a USB stick with the files into one of the front USB ports.

 

Or, plug the audio output from the MP3 player into the 1/8" audio input jack, you will control the songs that play on the MP3 player, and adjust the volume/tone on the car's audio system, but can't control the MP3 player form the dash panel.
Welcome to the 20s!

 

I have music files on my phone and during long road trips listen to the music on my phone through Android Auto. There are about 17000 music files, and they slow down the phone - when it was just a few hundred songs the phone was more responsive. Youtube music will show up in the Android Auto window, but the number of songs makes it easier to manage the music from the phone rather than Android Auto. It makes my life easier if I shuffle the music, that way it just keeps playing without me doing anything.

 

I have music files on my phone and during long road trips listen to the music on my phone through Android Auto.

I use PlexAmp via Android Auto. Ultimately plays from my server at home over the internet, but it will locally cache tracks on my personal playlists (without slowdown) or around 3-5 tracks around what you are currently playing so there's no dropouts from track to track.

 

User error.

 

Keep trying. There must be a simple reason your car audio does not pair to your device.
How old is this mp3 device? Will it connect to other Bluetooth devices?
As some else suggested, you can also fill a usb thumb drive with audio files and plug that into your usb port. I suggest a few 2gb drives to start. If you put too much on a drive the system is slow to organize and access. The files will be accessed in alpha numeric order also so you cannot play full albums in original sequence unless you rename the files. But shuffle play will be ok if you are just listening to random pop songs. I have even connected a small computer tablet to my Subie via Bluetooth just to experiment with its audio player.

 

You can get XM for $5/month, or free via the XM app if you already have an XM subscription on another car. The disadvantage of the app is it needs a cell signal, whereas an individual subscription for the car just needs to see the sky.

 

You should try a free account with Spotify. If you install the app on a laptop or tablet you get free access to entire albums in original track sequence. Many commercials as well but you can create playlists, listen to podcasts, profile auto generated playlists, all for no fees.
Now, the experience on a portable digital device such as a phone is different. You have limited access with a free account but you can still compile unlimited playlists.
Satellite radio has its place for some. Not for me but some. It’s like buying bottled water.
I am in a place where I have a respectable music collection and years ago I began my investment in also creating a library in iTunes. Importing records and compact discs into
iTunes libraries. I could then put my collection on iPods and iPhones. Also remaining loyal to having a cd player in the cars I drive, I have access to much of my library at anytime.

 

The Touring model I purposely bought had to have many options for listening to music. You can always and easily change your audio unit to include a cd player and other features but even with no cd player you have other options.
I have an iPod with 4,000 songs, an old iPhone with 9,000 songs. My main iPhone has 31,000 and still growing. I connect these through the usb port and CarPlay or Bluetooth. As I mention I even have a small tablet with audio files I have connected through Bluetooth. A fistful of 2gb thumb drives each with music collections of a variety of artists.
You have options.