If you use non workshop mods and want to be able to quickly install and remove them, you can do it with the Custom Workshop Installer.
https://steamhost.cn/steamcommunity_com/sharedfiles/filedetails/?id=1445498636

First of all make sure the mods have the right folder structure.
For example car mods need to be in cars\racers\ folders.

Then you need to put the mod in the workshop folder.
Warning! Steam removes everything in this folder when uninstalling the game, so don't store your mods there permanently.
(I will try to add the customizable directory to the Custom Workshop Installer later)

The workshop content for this game is downloaded to
"your steam library folder"\steamapps\workshop\content\497180
To find "your steam library" folder go to
Steam > Settings > Downloads >STEAM LIBRARY FOLDERS

Steps:

1. open "your steam library folder"\steamapps\workshop\content\497180
   
2. create a new folder and name it with digits only, for example 1000
   
3. put mod contents in this folder, retaining the game's structure - e.g. a car need to be inside cars\racers folder
   
4. a) create text file inside the mod folder and name it the same, e.g. 1000.txt,
   then open it, put the mod name at the first line and save it
   or
   b) add the mod name to the item names file - see below
   
5. run the Custom Workshop Installer and you will see the mod with WorkshopId 1
   
6. install the mod

*b)
Create the file "_CustomWorkshopInstaller_ItemNames_user"
and copy-paste the line below in the top of the file, like this (for example with Notepad++)