Due to changes in the original def files of the game, in the final version 1.31.x of American Truck Simulator the dolly cables have stopped working.
This is the fixed and corrected version.
Sorry for the inconvenience.
Tested in 1.31.x FINAL VERSION.
Credits: CyrusTheVirus, Siebel3D, MiguelAngel988, Crisan21, invasor