From a New York Times story on the decline of the cassette player:

Popping a cassette in the car tape deck is also passé. Only 4 percent of vehicles sold in the United States during the 2007 model year had factory-installed cassette players, according to Ward’s Automotive Yearbook. As recently as the 2005 model year, 23 percent of vehicles had them.

Given that the median age of a car in the United States is nine years old, said Alan K. Binder, the editor of Ward’s yearbook, it is most likely that the majority of the 200 million cars and light trucks on America’s roads have cassette players (though how many have had the same Bob Seger tape lodged unplayable in them for 11 years is impossible to determine).