It's not about the radio, these days the 'radio' is has a capacitive touch screen, GPS receiver, Bluetooth, HDMI/Wifi, a CPU that could have comfortably sent man to the moon in the 60's, cooling fans, amplified aerials, 6+ speakers, a sub and amp or two etc.
That's an not insignificant load when you add that all up. My 180W solar panel is just about enough to hold about 12.3v with the ignition and stereo on.
If you've got a single speaker AM unit, then that's different
