[Elsie]

My understanding is that the AM antenna has always been a small device within the radio, itself.

In a home radio - yes; but not in a car.

https://en.wikipedia.org/wiki/All_American_Five

[Empire_of_Liberty]

So, in a car, the AM antenna is incorporated into the FM antenna?

In the 70’s I had a Toyota that only came with AM. Maybe it was “FM Ready”, since it had an antenna.