How to add a trigger to a WPF custom control without overriding the existing style?

I am creating a simple custom control extending from toggle button that allows the user to specify checked and unchecked content directly in XAML. It works well but it is based on a trigger, and I don't know how to define the trigger except in a style. If I define the style, then I lose anything set outside of the custom control.

What I would like to be able to do is just append this trigger to any existing style set elsewhere on the control.

Here's the XAML for the style/trigger.

<ToggleButton.Style>
    <Style TargetType="{x:Type ToggleButton}" BasedOn="{StaticResource {x:Type ToggleButton}}">
        <Setter Property="Content" Value="{Binding RelativeSource={RelativeSource Self}, Path=UncheckedContent}" />
        <Style.Triggers>
            <Trigger Property="IsChecked" Value="True">
                <Setter Property="Content"
                        Value="{Binding RelativeSource={RelativeSource Self}, Path=CheckedContent}" />
            </Trigger>
        </Style.Triggers>
    </Style>
</ToggleButton.Style>

I tried inheriting the style via the BasedOn with a default type but it won't work if the custom control has an explicit style set by its parent. I also considered EventTriggers but I do not believe there would be an event to initialize the control.

Thanks for any help anyone can offer. :)