Interesting...
The Tesla website still lists AM radio as standard on a Model S.
If you don't have AM radio - then contact Tesla about getting the problem corrected.
Many (most?) AM stations have HD radio today. But the interface for HD radio is awful. If you're looking for a specific AM channel, unless you know the specific FM station and sub-channel, you may have to step through each FM station and each sub-channel until you can find the AM/HD radio station you are looking for.
Tesla could make this much easier with some improvements in the media player user interface. One possibility would be to automatically map the HD radio versions of the AM stations onto the AM user interface (which seemed to be what the user interface is doing, if you enable HD radio), so when you select a specific AM station that has an HD radio broadcast, you'd use the HD station automatically. And for the X & S that lack AM radio, the AM band would only provide the HD stations - and not be able to tune in those AM channels that are broadcast only in analog.