I just wanted to re-install only the AU component, because I deleted it accidentally. However the installer does not re-install the missing components, even though the product manager sees that it is not installed, and you get warnings that the AU component is not installed. So if you want the installer to re-install ALL components, then you really really need to delete ALL of the directories and files listed in the accepted answer, and only then run the installer. If even only one plugin remains(in my case I had forgotten /Library/Application Support/Avid/Audio/Plug-Ins/EZdrummer.aaxplugin) it will not re-install the components it seems.
