"
No 787 in log.txt.
Now original car works, I don´t understand.
Car's folder: "\common\assettocorsa\content\cars\Mazda_787b_ACi
787b works fine for me. Are you sure your mod car's proper name is "Mazda_787b_ACi"? The original is "ks_mazda_787" and it is case sensitive. Look in "%UserProfile%\Documents\Assetto Corsa\logs\log.txt" and Ctrl+F for 787; you should be able to find some reference to the internal name AC uses for the car. That's the name you need at the top of your json file.
No 787 in log.txt.
Now original car works, I don´t understand.
Car's folder: "\common\assettocorsa\content\cars\Mazda_787b_ACi