Cherry top (slang)
This article relies largely or entirely on a single source. (November 2007)
Cherry top is American slang which refers to a specific type of Police car with only red rotating emergency vehicle lighting mounted to the roof. The term originated in the 1960s, which is when these red rotating "beacon" lights were common. 
Most modern Police cars have more elaborate lighting systems than the traditional rotating light(s) and they emit multiple colors; however, the term "cherry" or "cherries" is still sometimes used to refer to any emergency lighting on a Police car. Michigan State Police continues to employ traditional red rotating beacons on its patrol cars.