Movistar (originally Telefonica) here in Spain offers a plug-in 4G router for car use - it plugs into the car's diagnostic port. It provides an android/iOs app which let's you monitor journeys/location of the car, etc.
BUT, it also adds 5 wifi access points for use within the car with a monnthly 10 GB data balance. All this for 3 euros a month.
Initially, I used this with an old android phone to play internet radio to the car's audio system. Result, but I find smartphones a pain to use, so I replaced the phone with a portable internet radio.
After many searches, I hit on the Ocean Digital WR-26 - small enough to fit in the car's storrage box between the seats. Runs on batterry or 5v 1 amp power. Fiddly to set up, with an old-style LCD monochrome display panel, but easy single button use including on/off and favourites. Nice sound from the stereo phono output.
The WR-26 is also an FM and DAB radio - both of which are useless/awful in Spain. So internet radio it is - BBC, etc. all available from the Skytune (?) aggregator. Addionally the radio has a built in http server database where you can easily add stream urls. The WR-26 feels and looks well-made. Comes with an extensive intelligable manual.
I got the OD from Amazon France for 59 euros.