To upload cars for Assetto Corsa, use either the File Manager guide or FTP Guide.
To enable them in game follow the Enable In Config guide.
File Manager Guide:
Stop your server.
Open your Game Panel, go to the File manager and browse to File Manager > \content\cars\
Once in this location create a new folder and name it the exact name of the car folder you have downloaded.
Go into this new folder you created, and upload the data.acd of the modded car you have downloaded.
Follow Enable In Config guide to add the car to your server.
Start server.
FTP Guide:
Here are our guides on basic FTP use.
What is FTP: https://streamline-servers.com/billing/knowledgebase/5/What-is-FTP.html
How to Access FTP: https://streamline-servers.com/billing/knowledgebase/29/TCAdmin-FTP.html
Stop your server.
Open your FTP program and login to the server.
Browse to the \content\cars\ directory.
Find car mod folder that you wish to use on the server, and upload it to this directory.
Follow Enable In Config guide to add the car to your server.
Start server.
Enable In Config:
Go to your Game Panel and open Configuration Files.
Open the server_cfg.ini.
Add the name of your car folder to the CARS= line and seperate each car with semicolons, eg CARS=abarth500_s1;ferrari_458
Make sure to add the cars as well on each entrants in entry_list.ini file.
Your car should now be enabled on your server.