I spent many hours on this one so please don't steal / repost it as your own.
To install the Atlanta Police Department Skin:
1) Open resources.assets with unity bundle extractor and replace Veh_Fex_01_exterior_dif_PD3 with this file.
If you'd like to add to the realism, check out my Georgia Plate mod here.
NOTE: Make sure to ONLY edit Texture files.