Custom Control Binding Issue

[Kevin Marois]

I am creating a Custom Control. Everything is working fine but I have one binding problem that I can't seem to get past. Here's the control's XAML. Some of the XAML has been removed for brevity.

    <Setter Property="Template">
        <Setter.Value>

            <ControlTemplate TargetType="{x:Type local:NavigationList}">

                <Expander Background="{TemplateBinding Background}"
                          BorderBrush="{TemplateBinding BorderBrush}"
                          BorderThickness="{TemplateBinding BorderThickness}"
                          IsExpanded="{TemplateBinding IsExpanded}"
                          HorizontalAlignment="{Binding HorizontalAlignment, RelativeSource={RelativeSource AncestorType=ContentPresenter}, Mode=OneWayToSource}">

                    <Grid>

                        <ListBox Grid.Row="1"
                                 ItemsSource="{Binding Items, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                                 SelectedItem="{Binding SelectedItem, RelativeSource={RelativeSource TemplatedParent}, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                                 BorderBrush="Transparent"
                                 HorizontalAlignment="Stretch">

                            <ListBox.ItemTemplate>
                                <DataTemplate>
                                    
                                    <StackPanel Orientation="Horizontal">

                                        <Image Source="{Binding ItemImage, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
                                               Width="16" 
                                               Height="16" 
                                               Margin="2"
                                               HorizontalAlignment="Left" 
                                               VerticalAlignment="Center"/>

                                        \*\*\*\*\*\* THE PROBLEM IS HERE \*\*\*\*\*\*
                                        <TextBlock M</x-turndown>