There should be some regulation in the radio itself on the DC input,
This doesn't have to be the case, but most radios provide a DC input
for use in mobile applications, and car voltages vary from a low if the
car is not running to a high if the battery is being charged. Even
worse would be spikes from a bad diode in the alternator. Generally a
device designed to work from auto power is designed to handle at least
16V. Many put the limit at 24V, which comes from some case where people
would put car batteries in series. Yeah, I know this sounds odd, but I
worked on a bid with Chrysler and they had a 24V upper limit on the
electronics at the time. The story I got was some people would jump
start cars with two batteries in series.

The DC input should be filtered a bit in the radio as well since auto
power isn't all that clean.