Multiplayer using object in scene

Hi everybody,

I am getting in touch with unity and play around with the standard assets. The goal is to create a multiplayer racing game. The player spawns as "Ethan" and can use cars parked around the terrain.

I am using all the standard assets: Ethan as Person, MultipurposeCameraRig, car, NetworkManager, etc.
Everything is working fine but I have problems letting the spawned player use a car already placed in the scene. I managed to do it in singleplayer but it doesn't work in multi.
My idea was to have a boxcollider on the car as trigger. The OnTriggerEnter does:
- change tags (Ethan = Untagged, car = Player)
- disable PlayerControlscript of Ethan
- enable CarControlscript of car
- change Target of Camera to car
- up to this point it works fine in singleplayer
- "put the car into the network" -> fails: car can not be controlled by player and many errors:
"NullReferenceException: Object reference not set to an instance of an object
UnityStandardAssets.Vehicles.Car.CarController.SteerHelper () (at Assets/Standard Assets/Vehicles/Car/Scripts/CarController.cs:273)"

it refers to that line of code:
m_Rigidbody.velocity = velRotation * m_Rigidbody.velocity;

It seems that my network-player-instance doesn't "know" about the rigidbody and cant't control it.

Maybe the simple question should be asked: How can I set a car already placed in the scene as network-player and let the player have control of it?

Following code is used:

Code (CSharp):

1. void OnTriggerEnter (Collider other)
2.     {
3.         if(other.gameObject.name == "PlayerPerson(Clone)")
4.         {
5.             NetworkIdentity oldPlayer = GameObject.FindGameObjectWithTag ("Player").GetComponent<NetworkIdentity> ();
6.             GameObject myCar = this.transform.root.gameObject;
7.             GameObject myPerson = other.gameObject;
8.  
9.             other.gameObject.tag = "Untagged";
10.             myCar.tag = "Player";
11.  
12.             GameObject myCameraRig = GameObject.Find ("MultipurposeCameraRig");
13.             AutoCam ac = myCameraRig.GetComponent<AutoCam>();
14.             ac.SetTarget (this.transform);
15.  
16.             CarUserControl carScript = myCar.GetComponent<CarUserControl> ();
17.             carScript.enabled = true;
18.  
19.             ThirdPersonUserControl PersonScript = other.gameObject.GetComponent<ThirdPersonUserControl> ();
20.             PersonScript.enabled = false;
21.             other.gameObject.SetActive (false);
22.  
23.             NetworkIdentity netident = myCar.GetComponent<NetworkIdentity> ();
24.  
25.             NetworkConnection conn = oldPlayer.connectionToClient;
26.             NetworkServer.ReplacePlayerForConnection(conn, myCar, 0);
27.         }
28.  
29.     }

I hope that someone is willing to help. Thanks in advance.

intonator