If you wish to have realistic flash patterns, enviromental lighting and proper headlight coloring for non-els fire trucks, ambulance and police cruisers ( sirensetting slot 1 and 2 ), install this. 

The proper headlight coloring will match the ELS headlight flashers. Rockstar had made the headlight to look cold, and so i increased the light coloring to warmer temperature (halogen.)

INSTALL PATH ( MAKE A BACK-UP FIRST ! ) : 

GTA V/update/update.rpf/x64/data 