I need to look at my code but from what I remember setup was like this:
Each of your actors - tractor and trailer, need to have a physics simulated component, attached to their root with physics constrain locked on all axis. This will be your couplers.
From tractor or trailer, spawn a physics constraint ACTOR not a component. Spawn location should be somewhere in between two objects that you want to constrain.
Both tractor and trailer implement interface which returns reference to their coupler components. This interface allows to plug both couplers (one from tractor and one from trailer) into Set Constraint Components of Physics Constraint Actor.
While I was writing this I’ve realized one unfortunate detail, this might not work with vehicles based on vehicle movement class. At least your trailer shouldn’t be based on vehicle class as they are purely kinematic and can’t be moved by physics interaction. Tractor should work fine as long as coupler is attached as child to kinematic parent.