Today's car radios are digital -- even the RF front end. And anything with HD Radio has I and Q (amplitude and phase) detectors, which means it would be trivial to implement synchronous detection to eliminate selective fading distortion, and also C-Quam AM Stereo (which many HD Radios secretly support). Noise blanking, too -- I had a 2007 Kia which was a lousy car, but its factory radio had noise blanking on AM, so when you drove under a noisy power line, there was no crackle or buzzing; the audio just got a bit distorted and choppy.