As usual I will now post crappy pictures that I took with my phone:
POWER!!

As you can see these things are bigger than a post-it note

You're going to want to buy some wiring and heat shrink and tape and zip-ties and butte connectors. Probably also some wiring conduit to tidy everything up.

The horns and relay have standard 1/4" spade terminals so you'll also want some female connections for hot wiring action.
Next you need to find somewhere to put these things. I could probably have just used the provided brackets and mounted them where the stock horns go. Mouting these things behind the grill is all the rage on nasioc so I decided to do that instead.
I scrounged through the scrap bins in the maching shop at work and found a nice piece of 4x4x1/4" aluminum out of which I cut two 3/4 strips. Then I sawed off the excess and milled the top down so that I could mount them flat against the bottom of the radiator support (which is angled).

I cut that groove out so it would fit up against the hood latch cable and milled down that little circle so that I could thread the bolt down on the back of the horn like so:

I also tapped threads into the bracket so I could just run a bolt through the rad support into them and not have to worry about holding a bolt or something. They're metric of course.

Here is how they mount:

I used a hole that was already in the support for that hole and then drilled new ones for the rest.

Then came time for the wiring. Heat shrink tubing is a good idea beacuse it keeps everything nice and sealed. I also cleaned up the wiring for my lights and put everything in conduit. It got kind of dark and I was working with a headlamp so I might take some pictures of that stuff tomorrow. To hook up the relay I found a thread about these things on nasioc that had some nice diagrams. Then I wrote the following on a post-it:
85 - Ground
30 - POWA
87- Horns
86- Horn tap
I used one of those vampire taps to connect to the existing horn wiring and the grounded it to the horn bracket with the existing horns. The new horns are powered and ground directly to the battery along with my lights.

You can see they hide quite nicely behind the lights. I don't think airflow to the radiator will be reduced much and I took a "spirited" drive with them mounted and didn't have a problem. Although this morning there was a mysterious small puddle of coolant on the ground that I will definitely need to look into.

I have only tapped the horns and I must say there were pretty loud. I would not want to piss me off.
Also I promise I don't sit at home Saturday nights and post on the Internet. Well, not most Saturdays. I actually went out with friends and saw Borat. Which was funny. Since I seem to be in the bonus section of this post I'd might as well post a bonus picture:

Yeah, that's a cougar painted onto the side of a landcruiser.