How to use the CommandParameter with a RelayCommand?

I am still learning the ropes of MVVM and WPF, and at the moment I am trying to create a Mediaplayer using MVVM. After intensive googling I decided that using CommanParameter would be the best way to avoid code behind. I reckon the code and XAML looks fine, but there is no magic- AKA nothing is happening.

Is there any kind soul out there who would mind have a look at my code and give me some advice? As always, I truly value yours answers. Please ignore my plurals in RelayCommands, it was getting late :)

XAML

<MediaElement Name="MediaElement" 
    Source="{Binding VideoToPlay}" 
    Width="400" Height="180" Stretch="Fill"
    LoadedBehavior="Manual" UnloadedBehavior="Manual"/>
<Slider Name="timelineSlider" Margin="5" Width="250" 
    HorizontalAlignment="Center"/>
<StackPanel Orientation="Horizontal" HorizontalAlignment="Center">
<Button 
    Command="{Binding PlayMediaCommand}" 
    CommandParameter="{Binding ElementName=MediaElement, Mode=OneWay}">&lt;&lt;</Button>

C#

class MediaPlayerViewModel: INotifyPropertyChanged
{
    private MediaElement MyMediaElement;

    private Uri _videoToPlay;

    public Uri VideoToPlay
    {
        get { return _videoToPlay; }
        set
        {
            _videoToPlay = value;
            OnPropertyChanged("VideoToPlay");
        }
    }

    void SetMedia()
    {
        OpenFileDialog dlg = new OpenFileDialog();
        dlg.InitialDirectory = "c:\\";
        dlg.Filter = "Media files (*.wmv)|*.wmv|All Files (*.*)|*.*";
        dlg.RestoreDirectory = true;

        if (dlg.ShowDialog() == true)
        {
            VideoToPlay = new Uri(dlg.FileName);

        }
    }

    RelayCommands _openFileDialogCommand;
    public ICommand OpenFileDialogCommand
    {
        get
        {
            if (_openFileDialogCommand == null)
            {
                _openFileDialogCommand = new RelayCommands(p => SetMedia(),
                    p => true);
            }
            return _openFileDialogCommand;
        }
    }

    RelayCommands _playMediaCommand;
    public ICommand PlayMediaCommand
    {
        get
        {
            if (_playMediaCommand == null)
            {
                _playMediaCommand = new RelayCommands(p => PlayMedia(p),
                    p => true);
            }
            return _playMediaCommand;
        }
    }

    void PlayMedia(object param)
    {
        var paramMediaElement = (MediaElement)param;
        MyMediaElement = paramMediaElement;
        MyMediaElement.Source = VideoToPlay;
        MyMediaElement.Play();
    }



    protected void OnPropertyChanged(string propertyname)
    {
        var handler = PropertyChanged;
        if (handler != null)
            handler(this, new PropertyChangedEventArgs(propertyname));
    }

    public event PropertyChangedEventHandler PropertyChanged;




class RelayCommands: ICommand
{
    private readonly Predicate<object> _canExecute;
    private readonly Action<object> _execute;

    public event EventHandler CanExecuteChanged;

    public RelayCommands(Action<object> execute)
        : this(execute, null)
    {}

    public RelayCommands(Action<object> execute,
                   Predicate<object> canExecute)
    {
        _execute = execute;
        _canExecute = canExecute;
    }

    public bool CanExecute(object parameter)
    {

        if (_canExecute == null)
        {
            return true;
        }

        return _canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

    public void RaiseCanExecuteChanged()
    {
        if (CanExecuteChanged != null)
        {
            CanExecuteChanged(this, EventArgs.Empty);
        }
    }

}