You could just use a WiFi hotspot in your car to make it work, the Echo Dot uses a regular USB cable for power, and the Bluetooth audio-out pairs to my Ford Sync audio just fine.
My car wifi is actually more complicated, I have a WD N900 “Central” version that is the main wifi attach point - it has a built in hard drive so I can stream multiple MP4 movies to tablets in the car. On the wan side of the N900 I have a MikroTik Metal wireless bridge that connects to my WiFi virgin mobile hotspot in the car (or when parked, to my house WiFi for uploads and downloads to the drive, or McDonald’s, or wherever I want to leach a WiFi signal from) to share with the rest of the car.