To install, use a program like OpenIV to access the GTA 5 files.

There are two methods of installing. You can either replace the entire
vehshare.ytd file with the new one provided here, or you can go into
the file and replace just the individual livery texture with the 
picture alone.

The vehshare.ytd file is located in 
Once that's done, go to GTAV/x64e/levels/gta5/vehicles.rpf

With the current limitations, this is around as close to getting an
NYPD vehicle skin for the police car until more modding tools
get released.

This texture was made by Wake and first released on July 26th 2015.