You can add extra cars. You just need a surgical approach. Follow this protocol:
Race Injection pushes rFactor to its 32-bit memory limit. Each extra car mod adds .MAS files (archives). Once you exceed ~4GB of virtual memory during loading, the game crashes.
Go to Documents\SimBin\RACE 07 . Delete the CustomSkins and UIData folders to prevent UI glitches.
Highly recommended for managing and updating existing mods and their IDs automatically.