Binding to a cutom property of the template parent

Q: How do I bind to a custom property of the template parent from a child control's style DataTrigger

I've been scratching my head over this one for a couple of days.

I have a databound TreeView which uses a Style which has a Template. The TreeView is bound to a ObservableCollection and a HierarchicalDataTemplate + DataTemplate bind to properties inside a collection item.

FontGroup -> Font(s)

<Style x:Key="ExpandCollapseToggleStyle" TargetType="{x:Type ToggleButton}">
...
<Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="{x:Type ToggleButton}">
                <Image x:Name="ExpanderImage" Source="/Typesee;component/Resources/tree_expand.png" RenderOptions.BitmapScalingMode="NearestNeighbor" />
                <ControlTemplate.Triggers>
                    <DataTrigger Binding="??? IsItemSelected ???" Value="True">
                            <Setter TargetName="ExpanderImage" Property="Source" Value="/Typesee;component/Resources/tree_collapse_selected.png" />
                    </DataTrigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

<ControlTemplate x:Key="FontTreeViewTemplate" TargetType="{x:Type TreeViewItem}">
...
    <ToggleButton x:Name="Expander" Style="{StaticResource ExpandCollapseToggleStyle}" ... />
...
    <ControlTemplate.Triggers>
            <DataTrigger Binding="{Binding IsItemSelected}" Value="True">
            <!-- WORKS FINE HERE -->
            </DataTrigger>
    </ControlTemplate.Triggers>
</ControlTemplate>

First I tried to bind like:

Binding Path=IsItemSelected, RelativeSource={RelativeSource TemplatedParent}

Then I read that might not work so I tried (including AncestorLevel 1+3):

Binding RelativeSource={RelativeSource Mode=FindAncestor, AncestorType={x:Type TreeViewItem}, AncestorLevel=2

Have also tried combos with UpdateSourceTrigger=PropertyChanged and Mode=TwoWay

If this is a flawed design please suggest a way of doing this: I basically want to change the image of the expand toggle button based on whether the property IsItemSelected is true on the TreeViewItem -- any ideas?

Thanks so much for any help!

The viewmodel in all likelihood will be the DataContext, so the binding should be a RelativeSource binding with a respective path which needs to explicity target the DataContext as the new source is the RelativeSource:

 {Binding DataContext.IsItemSelected,
          RelativeSource={RelativeSource AncestorType=TreeViewItem}}

As noted in my comment it might be advisable to extract this logic from the ControlTemplate as this leaves its bounds. One method would be subclassing the ToggleButton and exposing a public property for the image which then can be changed via a Style.