Best way to enjoy my digital hoard of audio books

Here is a rundown of laws regarding headsets/earphones by state

I would simply replace the non-working radio with a new one that includes an auxiliary input, USB port, etc. Then you can just hook up your pi and start listening. Keep the speakers for that radio to the cab area and you won't bother everyone in the back. You can get a new radio for $100 or less installed.