DC-DC isolation for my car stereo... Where do I start?
Headlights are DC. So I'd guess that the alternator is noisy [under load], and your stereo is picking that up.
You could try beating the noise at the source, or focus on the stereo. If you want to rip off &amp; re-terminate your stereo DC plug, you could wind a couple of turns on a toroid [ring] core. If playing with plugs doesn't appeal, try winding the DC cable around a ferrite rod. Hold it in place with electrician's plastic insulating tape.