you may have noticed that few modern cars have any visible AM/FM antennas these days. Many have a "shark fin" antenna on the roof for GPS and possibly 4G cell phones - frequencies used are similar. Others have a short whip antenna somewhere, not much good for AM.
The answer ( found by Googling) is they often use the rear window "defroster" grid for AM and FM reception. It can be made to work like a frame antenna for AM and a L shaped wire for FM. A booster amp is added near the window for impedance matching.
Some makers ( BMW ?) have two printed antennas on rear side windows that are boosted and fed into a selector unit that sends the stronger one to the receiver. This is called "antenna diversity" as is commonly used with radio mics.
If anyone knows more, make my day.