WoWInterface - UIParent HookScript for OnMouseDown not working

Hey guys.

EDIT: This is for wow classic! (sorry)

I was hoping to use HookScript to handle "OnMouseDown" events for the entire UI globally. If there's a better way to do this, I'd also be interested!

If I use SetScript, it does in fact work for my purpose but it breaks behaviour in the default UI as I imagine it's overwriting an existing "OnMouseDown". So I tried HookScript instead but the event is never handled.

Code:

 UIParent:HookScript("OnMouseDown", function(self)

        print("click"); -- doesn't work!

    end)

Code:

UIParent:SetScript("OnMouseDown", function(self)

        print("click"); -- works! but breaks default behaviour...

    end)

Any ideas?