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Why is not the file there?
If you have followed the instructions of the mod manual to do a standalone installation probably you has deleted all the content inside rFactor/Gamedata/Vehicles except the HISTORIC GT folder.
Sadly there was a mistake on that part of the manual, and only the folders except Historic GT's should be deleted, not the files.
My most sincere apologies!
How to fix it
Just copy that MULTICAR.MAS file from other rFactor installation to the rFactor/Gamedata/Vehicles folder on the standalone installation.
(Thanks to Pain-less for the tip)