Critical

Hey there!
For whatever reason, searching for model names in OpenIV does not work.  You have to go find the models yourself.  The police models can be found in the patchdays, as well as x64e.  As a reminder, use a mods folder!  I use a template carvar for all my standalone police cars I add, and if you'd like to use it, here - http://pastebin.com/vvxCKdN8
As per siren settings, there quite a few, but you're normally only  going to be using siren setting 1, unless the vehicle is a bike, firetruck, etc.  Try playing around with the siren settings to see which one works best for your car.  If none work, then report back and we'll continue trying to figure it out.
 

Well, there should be atleast one there... Hm. Try googling to see if you can find out where they are. Or, just download one of BxBugs' vehicles and use his carvariations, i guess. 

For something like this, Youtube is the place to show you where to find carvariations, and how to edit them. I will try my best to help you out.
1) Locate the carvariations for the car you are modding. For example, if you installed a police car that didnt come with them, look into the dlc packs (Not patchdays, the other ones), drag the carvariations to your desktop, hit Ctrl F and type in Police, then search. To find the carvariation files: GTA V > mods > update > x64 > dlcpacks > any pack between mpapartment and mpvalentines2 should have carvariations > dlc.rpf > common > data > drag carvariations to desktop
2) That will most likely take a few minutes, keep looking. Once you find it, scroll down until you see Siren Settings (Should be the last thing you see before the start of a different vehicle) (By last thing, i dont mean scroll all the way down to the bottom of the file. Different cop car settings are in the same file. <Item> separates the different cars) (That was probably confusing. Sorry if it was)
3) Most cop cars use siren settings 1 and 2. So, in the siren settings you just found, change the siren setting to 1. Then, go back into open iv, hit edit, and drag the modified carvariations into open iv. 
4) Go in game. If the file works, great. If not, change the siren settings to 2. 
5) If 1 or 2 doesnt work, well have fun looking through all the siren settings. I think it goes up to 20.. Not 100% sure though.
If this doesnt help, just search on youtube how to edit carvariations file in gta v. Hopefully a video there can help you.