Good question.
IMO, AM Stereo has some somebody, somewhere, laughing their (rich) butt off. What a stupid idea. AM is the realm of talk shows, not music, so why stereo? All that it's done is make it a lot more difficult to listen to a weak station. Today's car radios have pretty lousy sensitivity on the AM band anyway. The extra cost for stereo would be much better spent on a better RF section. I miss the old Delcos from the '60s with the 5-gang ferrite tuner sections. Those were some great receivers. I even have difficulting receiving the local 50KW AM station from some parts of town, and have repeatedly not been able to hear it at all from ~60 miles away out of town, yet an FM station at the same location would come in loud and clear. Go figure.
I'd sure like to turn off the stereo on the AM, but I don't know of anyway to do that.