I am looking for an MP3 player that runs on batteries and rides with me in the car and plays through my radio/cassette player with a cassette transducer.
I am just getting back from days without internet with weird issues with my ISP. They said it was my modem and sent me a new one. Now my VPN box won't let me access the internet and I have to go straight from the modem to get online. GRRRrRRRRrrrrr...
The device is called a "smart phone".
And..........there's an app for that - in regards to your MP3 player running through your car stereo.
You're looking to marry new tech and old tech. Not likely to happen with a simple workaround. I've checked into it myself. There are some complicated schemes which require alterations to your stereo's wiring harness. This step will allow a blue tooth connection between your phone and the car stereo.
You could search for the wiring diagram of your particular car stereo and figure out how to "hack" a bluetooth connection into the audio input signal.