I don't know how to tap into a blank spot, but I use these. The original circuit has a fuse, the add on has a seperate fuse. Same as using a blank spot.
Very easy to connect.
I drilled a hole in the side of the fuse cover and threaded the new wire through with a spot of hot glue to seal the new hole.
I think. I have a picture in the axis Vegas 123 thread.
I tapped into the fuse block in the bin under your driver's seat.
The same tap(check fuse size before buying) will work on the under dash box.