Car can move backwards and sideways but not forward. also an audio problem

Hi, just bought a Car Model from the Asset Store and tried to make it working. I thought it was a good idea to just copypaste the script from the Standard Asset Car. And after some inital problems I can now get the car moving but only backwards and sideways. I don't get a compiler error so I have no idea what the problem might be and what infos I need to give you to help me. I'm new to unity and C++. Edit: I also found out that the brake doesn't function.

Also, I have one smaller problem with the audio of the car. I get the following compiler error:

NullReferenceException: Object reference not set to an instance of an object UnityStandardAssets.Vehicles.Car.WheelEffects.EmitTyreSmoke () (at Assets/Standard Assets/Vehicles/Car/Scripts/WheelEffects.cs:47) UnityStandardAssets.Vehicles.Car.CarController.CheckForWheelSpin () (at Assets/Standard Assets/Vehicles/Car/Scripts/CarController.cs:283) UnityStandardAssets.Vehicles.Car.CarController.Move (Single steering, Single accel, Single footbrake, Single handbrake) (at Assets/Standard Assets/Vehicles/Car/Scripts/CarController.cs:171) UnityStandardAssets.Vehicles.Car.CarUserControl.FixedUpdate () (at Assets/Standard Assets/Vehicles/Car/Scripts/CarUserControl.cs:27)

The script belonging to it (line 283 is the one with EmitTyreSmoke)

             "if (Mathf.Abs(wheelHit.forwardSlip) >= m_SlipLimit || Mathf.Abs(wheelHit.sidewaysSlip) >= m_SlipLimit)
             {
                 m_WheelEffects[i].EmitTyreSmoke();

                 // avoiding all four tires screeching at the same time
                 // if they do it can lead to some strange audio artefacts
                 if (!AnySkidSoundPlaying())
                 {
                     m_WheelEffects[i].PlayAudio();
                 }
                 continue;
             }"

Any help appreciated