Ah ha! I got the Truck Pack AI Traffic mod to work with ETS2 1.3.1!
The original instructions were a bit confusing. Why they didn’t include a Def file with it I really don’t know.
Copy this file I made which also allows for easier editing of the truck_traffic_storage.sii too!
See attached image for example.
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Version 1.3.1 tested Euro Truck
Author: Fabio Brazuca