To install the addon version of this car, follow the steps listed underneath.

Step 1.
Go to GTAV/mods/update/x64/dlcpacks and drag the folder golf8gti into it.

Step 2.
Go to GTAV/mods/update/update.rpf/common/data and edit the file dlclist.xml
In the file dlclist.xml, add a new line at the bottom: 
<Item>dlcpacks:\golf8gti\</Item>

To spawn the car, open a trainer of choice in game and spawn in golf8gti.

Enjoy!