- First you need internet in your car. That's a 3G device (unless WiFi is available everywhere you drive). That's a smartphone or a tablet.
- Then you need some program that can connect to Internet and play music. That's an app, unless it's hardware. Apps run on smartphones or tablets.
http://www.alibaba.com/product-gs/686440214/7inch_with_Linux_driver_3g_internet.html is one, although I don't see why a 3G-iPad wouldn't work.
If not happy with the sound quality of your device you need a connection between it and your car audio. That can be bluetooth or a cable, like you use to connect mp3-player to your car audio.
If it's not available in one box with the same form factor as a car audio, well, that''s more a question of marketing and sales predictions then a technical issue. And if it's isn't available, you can't buy it, whatever your expectations. So it's time to contact the makers of car audio systems and suggest them to start making it. They will have at least one customer!