That's been my experience too. As long as you had a good ground on the antenna housing, a coat hanger mast would work just fine. Since close to 100% of all cars just receive and not transmit through their antenna, it sounds to me to be a total nothing burger regarding ground planes and other technical stuff. I remember when I had my fiberglass body dune buggy with not only an AM/FM radio, but, a CB as well. I had two different antennas over the years. The first was just a regular telescoping tri-band type that came with the CB package. It was just mounted on the "hood" section of the body. No metal anywhere close to it. Worked just fine as long as I had a ground wire attached to it. For safety reasons, I changed to a lighted rear whip antenna for the sand dunes. It was mount in the rear section of the fiberglass body (no metal there either) and also had a power wire for the light on top. Worked fine there too. Now, could it have been tweaked to the nth degree by using a better location for optimal transmission and receiving? I would imagine so, but, I wasn't relying on it to defend a country.