Custom Renderer in external DLL not working

Hi,
I have a custom control & renderer (based on WP's checkbox) that I want to re-use. So I set up a PCL with the control definition and a project with renderer implementation. All works well, with data binding and events and stuff, but...
When I put the renderer file into a separate project/library and reference it from the main WP project, the rendering code doesn't execute. If I keep the renderer file within the same project (I'm currently keeping the file there as a link), all works well.
I must be doing something wrong - after all, the xamarin's platform DLLs are basically renderers, so the concept must be doable. So, what's the trick?
P.