There used to be the cassettes with a wire that would plug into a phone or a portable CD player, I used one for years in my old 79 K20.
Not sure if anyone still makes one or not.
I've used those before, but they were always buggy and always crapped out before too long. I'd probably be best off getting a better bluetooth speaker than my current little wonder boom. It gets overwhelmed by the road noise on the highway. I'm fine with setting my phone in the cup holder otherwise.