I did this mod today using the Fiamm El Grande horns - price is currently down to $14.22 at Amazon. In spite of all the excellent instructions and pictures, I still managed to make 2 rookie mistakes. Hopefully the next person can learn from them:
1. When I attached the included mounting brackets to the horns, I didn't realize that the two holes in each bracket are different sizes. The smaller hole needs to be used on the horn side because the existing bolts in the car only fit through the larger hole.
2. This is the biggie: my plan was to go in through the top because I don't have ramps or equivalent to raise up the front of the car. I made note of all the warnings about dropping a bolt into the abyss, but didn't think about this when removing the pop fasteners. Not one, but two of them went flying down into no man's land below. Uh oh, time to replan!
So I got down on the ground and removed the underbody panel to get those rogue fasteners out. Luckily I'm skinny and have long arms...
I removed all of the underbody fasteners and all bolts but the one in the rear. The first fastener from above was lying right on the panel, but the second required a little hunting: turns out it had bounced into the front bumper.
At this point, it was clear that even without ramps, going in through the bottom was going to be easier. The rest of the install was pretty straight forward, and the sound is 100% better now!
I took lots of pictures as well as before and after video. Will try to post them in the next couple of days.
Thanks to everyone who posted instructions and tips!