Most of my on-air time for the dozen years (1972-82) I worked at my hometown radio station was on the AM station. In the 60s (?) the station sold (at cost?) “FM converters” because so few cars had FM receivers. This NY Times story tells the tale:

“In 1978 half of all radio listening was on the AM dial. By 2011 AM listenership had fallen to 15 percent, or an average of 3.1 million people, according to a survey by Veronis Suhler Stevenson, a private investment firm. While the number of FM listeners has declined, too, they still averaged 18 million in 2011. (The figures are averages based on measuring listeners every 15 minutes.)”

“In 1970 AM accounted for 63 percent of broadcast radio stations, but now it accounts for 21 percent, or 4,900 outlets, according to Arbitron. FM accounts for 44 percent, or 10,200 stations. About 35 percent of stations stream content online.”

“Nearly all English-language AM stations have given up playing music, and even a third of the 30 Major League Baseball teams now broadcast on FM. AM, however, remains the realm of conservative talk radio, including roughly 80 percent of the 600 radio stations that carry Rush Limbaugh.”