Is AM listening in automobiles where the majority of AM radio listeners tune in? I would presume it's a big part of the pie. You can't hardly find a new after market radio to pick up AM. I know because I've looked and bookmarked the ones I find.
@centinel I had always presumed the car radio as the primary way most listen to AM today, mainly because I rarely see anyone using portable radios anymore. A couple years ago I went into Walmart just to buy a transistor AM/FM radio.. they didnt have any and seemed a little puzzled over my request when I asked what isle they were in. Finally one of their eyes lit up and they showed me a Dewalt worksite radio that was like $80 or so.. I dont know of any store you can walk into and buy a portable AM radio. They're easily found online though.
I'm not sure what you mean about not being able to find an after market car radio that has AM. They appear to be plentiful from a quick search.