I did this today. I bought these for $20 from Checkers:
http://www.oreillyauto.com/site/c/detai ... &ppt=C0107
My horns each had their own power and ground male faston connections. It also came with short jumpers that you could ground one terminal to the mounting screw. I used those but I also wanted to connect the OEM ground... so I remove the Faston tab from the OEM horn's ground and put it on the bracket of one of the horns which allowed me to connect the OEM ground to the horn's bracket. I had to drill out the hole in the tab slightly.
I did go in from the top ( I was lazy and didn't want to go under the car), but I didn't remove the reservoir. I unbolted it and flipped it to on top of the radiator support. No need to remove the hoses.
I bolted the two horn brackets directly to the main bolt, not to the OEM horn bracket. i.e. I removed the OEM horn and bracket. I felt the bracket(s) would be flapping in the wind too much. Though I'll say the brackets in the OP's pictures look much better than the ones that came with my kit. That still doesn't change the fact that the OEM horn bracket is pretty flimsy.
The most interesting thing to me was that the horn is NOT used for the noise that gets made when you press the keyfob lock button. I was expecting the noise to change but apparently there's yet another horn somewhere that makes the muted sneeze sound.