Go to a JY and plan on spending some time there. You'll need to get the fuse block out far enough to take it apart/mess with it. likely it's hardwired to the interior wiring, instead of being a screw type connection like the underhood box.
2003 SSEi added to his, and when I pulled an underhood apart from a W body..it was sealed w/o any way to do it. You might be luckier with the interior box. But go pull a couple apart and find out what makes them tick (I have one at the house actually.. hmm) then you could use pins from a JY car etc and add your circuits that way.
