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. [1]
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.
Example of usage: "I didn't realize how fast I was driving until I saw the cherries in my rear-view mirror."
References
Cherry top (slang) Wikipedia(Text) CC BY-SA