So I tried implementing something like this in the jigLib game.
Vector3 position = new Vector3(carObject.PhysicsBody.Position.X- 4, -14, carObject.PhysicsBody.Position.Z);
BoxObject obj = new BoxObject(this, boxModel, new Vector3(1), Matrix.Identity, position);
ConstraintMaxDistance constraint = new ConstraintMaxDistance(carObject.PhysicsBody, carObject.PhysicsBody.Position, obj.PhysicsBody, obj.PhysicsBody.Position, 0.2f);
So this is what I used. And I changed the materialProperties of the box to NormalNotBouncy and the mass to 10.0 . But the Behavior of the models are extremely erratic. As the box and the car spawn the box spins out of control sometimes pulling the car with it. If I lower the mass of the box it never even touches the ground. And if the cars velocity is high enough both the box and the car keep on spinning endlessly. Does anyone have more experience with the constraints and could enlighten me as to why this is happening?