User defaults

I managed to adjust register plus plugin to allow registrations of new users to individual blogs on MU. So now I can have people register to get a new blog via wp-signup.php or have them register as individual users on individual blogs via wp--login.php?action=register

Here is my problem.

I want the new users who register for individual blogs to automatically be set as authors.

I know that blog administrator can select a user (one at a time) and set that selected user as author (however that is useless to me)

Where in WP files is this setting defined as default. I am ok with having to change the core files (since I can't find the other way to do this).. I assume it would take changing something that says 'subscriber' to 'author'. All help would be greatly appreciated

(user defaults plugin does not work for this, I tried)