To install a car mod you need to do the following:
1 - Download the bootfiles and put them I'm the Mods folder.
2 - Go to bootfiles->vehicles->vehiclelist file and ADD you car crd entry at the end of the file
3 - in the same vehicles folder, go to physics->driveline and edit the driveline end of file
(Both vehiclelist and driveline entries should be included with the car you downloaded)
Then put your car folder in the mods folder
Then with jsgme
1- activate cars
2- activate bootfiles
You should be good to go