I use something like (not this exact model )
https://www.amazon.co.uk/Audio-Integ.../dp/B00H7AEE9Y
GROM Audio BT3 Integrated Bluetooth Car Kit
It has Bluetooth and you can plug in a usb memory stick or a memory card full of MP3's.
Then you can change folders the same way you would change cd's if you had a CD changer fitted and then use the normal track controls to change tracks via the steering wheel etc.
The version I have cost me about £35 from China.