Whoami, apparently he's asking how to make a siren mod, not how to make a car add-on as cop type.
Make a siren mod is really easy by using an already made sirens mod as base, use
this one. Extract the content in the .ar file by opening it with WinRar, do your edits to the .wav files, don't rename the files, and make the .ar file of all and try in-game. In MM2 the sirens work in base of the city, in the case of that mod it's for SF; from the .csv files can be set other ways of using the .wav files in-game.