Installing mods
Steam
You may browse the Steam Workshop here!
Non-Steam
However the mods are obtained, either via Nexus Mods, ModDB or some alternative place, all mods can be installed in your games in your game save folder.
Make a new folder called mods under %localappdata%\KeepDriving\ (Or alternatively C:\Users\USERNAME\AppData\Local\KeepDriving\), if one doesn't exist.

Then extract any mods and put them in the mods folder, under their own folder. And be sure that all of the kdr files are in the root folder of said mod folder.
i.e.
\KeepDriving\mods\mod_folder_here\*.kdr