INSTALLATION:

1.Copy "fiat600" folder to :\Grand Theft Auto V\mods\update\x64\dlcpacks

2. Using OpenIV, go to: \Grand Theft Auto V\\mods\update\update.rpf\common\data  find the "dlclist.xml"
and add the following line:

<Item>dlcpacks:\fiat600\</Item>

Done!
spawn: (cars -> Compacts ->) FIAT 600

NOTE: Tuning for this car are linked to Livery mods and would work with "Benny's Lowrider Mods". 
      Will not work for some very old outdated versions of the game.