Without knowing the details of what you changed or the navigation system API I'm not able to say why it's not calling SetDestination.
To get started I recommend that you do not use the Movement Pack and just get a basic task working from scratch. This will better show you the flow of creating a new task, and you can then be sure that you can get something working. Just start with having the vehicle move to a static point in the scene and expand from there.
After you have a handle on that you can implement the abstract Movement class within the Movement pack similar to what the NavMeshMovement class does and then create your own Seek, Wander, etc classes. You should not just modify the included NavMesh Seek task as that is not setup to use your own navigation system.
To get started I recommend that you do not use the Movement Pack and just get a basic task working from scratch. This will better show you the flow of creating a new task, and you can then be sure that you can get something working. Just start with having the vehicle move to a static point in the scene and expand from there.
After you have a handle on that you can implement the abstract Movement class within the Movement pack similar to what the NavMeshMovement class does and then create your own Seek, Wander, etc classes. You should not just modify the included NavMesh Seek task as that is not setup to use your own navigation system.