Adding Horn to '04 Toyota Highlander?

I have a question in regard to adding an additional horn to my Toyota Highlander (http://www.carforums.net/showthread.php?t=43563#).

Disclaimer: I'm new to the forum and I'm not a mechanic by any stretch of the imagination, but could claim to be a little bit of a handy man.

I bought an "Ooga Horn" for my wife's car for the fun of it and need to install it. The instructions seem fairly simple (in a nutshell):
1. Connect the positive to the battery with an inline fuse.
2. Connect the horn switch to the negative that "loops" to the ground.

Questions:
1. Can I hook up the "positive" end of the horn to an empty slot in the fuse box? Is power run to every terminal in the box, it's just a matter of connecting my wire to the fuse box?
2. IF I CAN do that, is it hard to get into the fuse box? I looked at the box briefly but couldn't determine that at first glance.
3. How difficult is it to fish a wire into the car for a switch and where should I ground the cable? Are there switches made to fit the empty slots in the dashboard?

Thanks for the help!
Ben