How Do You Turn on the Siren?

I've clicked just about every button on my keyboard and checked the vehicle controls, but i still can't find where the button for the siren is. Thanks in advance

 

Alright, I got the police cars to work, but when I went to do the ambulance, it won't work. I tried it on just the regular, default ambulance and nothing happened... Am I missing something?

 

Go under Roof Accessory in the Parts Menu...you should see a part that makes the siren sound...if it's not present, you have to add it to the vehicle.

 

Okay, thank you! I really have no idea why I'm having so much trouble figuring stuff out today. I blame Thanksgiving.