DLCPACK:
1. Go to Grand Theft Auto V\mods\update\x64\dlcpacks
2. Drop the `CityPoliceCar` folder in there
3. Go to Grand Theft Auto V\mods\update\update.rpf\common\data
4. Open dlclist.xml
5. Add <Item>dlcpacks:/CityPoliceCar/</Item> to the list

DLS XML:
1. Go to Grand Theft Auto V\Plugins\DLS
2. Drop the `transit1` XML in there