If you have the Gavril LED Flashers mod installed be sure you have version 3.02. Both mods have been updated to fix issues with the flat light bar after the v0.11 hotfix.
A hundred and twenty six so far and more to come!
Default sirens can be changed by clicking on the right arrrow next to Light bar in Vehicle Customization.
If you experience any issues like thumbnails not showing try clearing the cache.
Save the zip file to Documents\BeamNG.drive\mods
@KZrT010 for the Hungarian skin
@Duster02 for the Serbian Sunburst skin and help with adding light bars and beacon
@372minecraftlover for changing many of the sirens to make them more accurate.
Global Police Skin Pack 1.37.1
Police skins from around the world