I’m no developer but I think I’ve got the gist of it here:
Sometimes when you install a police car into FiveM you may notice an issue: Broken lights. There are different stages of severity: sometimes the lights won’t even turn on at all, and the siren won’t work. Sometimes the lights will turn on but they’ll be in weird positions, or it will just be a couple emissive in weird parts of the car. This is usually an issue of your siren ID. This unique ID is given to an emergency vehicle, but when another resource has the same siren ID, it will cause this issue. This happens more often than you would expect, and the solution is simply to change it.
Go to your carcol.meta and carvariations.meta files and open them both up. There you will find something that says “sirenSettings:” followed by a number (see photos). Change the number to a random six digit integer. Make sure the number is the same in both files. Save the file and upload it to your server, and restart. It may not work on the first try, but keep going at it. Change the number to something different and random each time, generally the higher the digits, the more success you will have. Eventually it will work, and then you have fixed your issue. Hope this works for you guys.