How about this?

// Set this up in the inspector
public Collider car;
bool paused = false;
void OnTriggerEnter(Collider other)
{
    if(other == car)
    {
        paused = true;
        Time.timeScale = 0;
    }
}

void OnGUI
{
    if(paused)
    {
        GUILayout.FlexibleSpace();
        GUILayout.BeginVertical();
        GUILayout.FlexibleSpace();
        if(GUILayout.Button("Unpause")
        {
            Time.timeScale = 1;
            paused = false;
        }
        GUILayout.FlexibleSpace();
        GUILayout.EndVertical();
        GUILayout.FlexibleSpace();
    }
}