I have a style that I am using to make dynamic tooltips on certain text boxes like so.

<Style TargetType="{x:Type TextBox}">
  <Setter Property="MinWidth" Value="100"/>
  <Style.Triggers>
    <Trigger Property="Validation.HasError" Value="True">

      <!-- item of interest -->
      <Setter Property="ToolTip">
        <Setter.Value>
          <MultiBinding Converter="{StaticResource ErrorMessageConverter}">
            <Binding RelativeSource="{RelativeSource Self}" Path="Tag"/>
          </MultiBinding>
        </Setter.Value>
      </Setter>

    </Trigger>
  </Style.Triggers>
</Style>

This works very well, but if I want to use a more complex tooltip I can't figure out how to bind to 'Tag' anymore for the converter value. For example;

...
<Setter Property="ToolTip">
  <Setter.Value>
    <StackPanel>
      <TextBlock>
        <TextBlock.Text>
          <MultiBinding Converter="{StaticResource ErrorMessageConverter}">

            <!-- item of interest -->
            <Binding RelativeSource=" what goes here?? "/>
          </MultiBinding>
        </TextBlock.Text>
      </TextBlock>
      <Image/>
    </StackPanel>
  </Setter.Value>
</Setter>
...

I have tried several flavors of 'FindAncestor' and what not for the relative source, but I can't get anything to work. Any ideas??

UPDATE: 12-29-2010 : Here is the correct code, answer provided by our friend Goblin below. Works perfectly!

...
<Setter Property="ToolTip">
  <Setter.Value>
    <!-- Item of interest -->
      <ToolTip DataContext="{Binding Path=PlacementTarget, RelativeSource={x:Static RelativeSource.Self}}">
      <StackPanel>
        <Image/>
        <TextBlock>
          <TextBlock.Text>
            <MultiBinding Converter="{StaticResource ErrorMessageConverter}">
              <Binding Path="Tag"/>
            </MultiBinding>
          </TextBlock.Text>
        </TextBlock>
      </StackPanel>
    </ToolTip>
  </Setter.Value>
</Setter>
...