Vista - pack URI format & resourcedictionaries

I'm trying to merge a resource dictionary with one stored in another
assembly in my project, but I cannot seem to get the URI format correct.
I'm not finding a whole lot of documentation on the format (at least for
this scenario; there's a lot of stuff referencing .jpg's, though), and I
haven't found a combination of settings that works.
Given:
Application.exe and Library.dll, I have a .xaml-based ResourceDictionary at
/Source/Resources/Resource.xaml, and MyApp.xaml tries to merge in:
<ResourceDictionary Source="{0}"/>

I've tried building Resource.xaml as EmbeddedResource, Content, Resource,
and Page (the default setting).
I've tried setting {0} to:
Resource.xaml
Resources/Resource.xaml
/Source/Resources/Resource.xaml
/Resource.xaml
pack://application:,Resource.xaml (says the URI prefix is not recognized)
pack://application:,Source,Resources,Resource.xaml
pack://siteoforigin:,Resource.xaml
pack://siteoforigin:,Source,Resources,Resource.xaml

None of these combos work, so I'm stuck. Has anyone had luck getting this
particular scenario working?

I got a bit closer in that the authority is working:
pack://application:,,,

but no matter how I package and reference that xaml file, it isn't finding
it.