I just installed mine yesterday (Pat Walsh bracket and Stebel Nautilus airhorn) and had no issues, except for mounting the bracket backwards the first time:
Here's the bracket alone:
Notice in the top image, the bend in the bracket is away from the horn. You want it the other way so that it is facing the horn and the nut is to the outside of the bike.
You do not need to drill any holes, as you will see in the picture, you are using the Nautilus stock mounting position, bolt, and plastic block that came with the horn.
Here's a walk-through of the entire install -
The hardest part is getting the fairing off, particularly if you have the Givi engine guards installed. First, remove the eight push-pin style rivets that hold the front (black) cowling on. The way to do this is to use a small screwdriver and push the center pin in ONE click, as shown:
This will let the pin "unspread" in the hole for removal. Don't lose the center pins as they will drop out on the floor, rolling under the nearest heavy, fixed object.
Next, remove the philips head bolts at the front of the cowling in the upper outside corners.
Remove the 4mm hex cap bolts at the top rear of the tank and along the sides of the left and right fairing. You need to remove the right side as well to get to the horn leads, which is behind the radiator. With the front cowling loose, you can get to the two philips screws (one on each side of the bike) that hold the front of each plastic fairing in place. Your V-Strom should now look like this:
Note where my finger is pointed - that is the brace behind the left headlight that you will be attaching the horn mount to.
Next, take your horn with the mounting bracket already attached (but loose, finger tight) and fit it around the headlamp brace. Insert the carriage bolt from the back, and install the lock nut (lock nut towards the outside of the bike). Begin to tighten the nut so that it is firm, but you can still "wiggle" it a bit on the brace. tighten the horn to the bracket the same way. It should now look like this:
Now you may want to test fit replacing the left fairing. You'll probably find that you are going to have to wiggle the horn around a bit. I actually needed to "twist" the bracket downwards a bit and push inwards about 1/2" for it to clear the fairing, but it does fit. Once you have it about right, tighten down all the nuts. It should now be looking like this from a 3/4 view:
I won't go into the schematics as those are covered in other posts (hey, use the search). Now you will need to remove the upper and lower radiator mount bolts (10mm wrench). Slide the radiator off it's mounting pin and you should be able to get your hand in there between the rear of the radiator and the fan shroud and pull the horn power leads from the stock horn.
Once that's done, remove the connector from the fan shroud. It's the whitish-yellow connector that is held to the shroud with a plastic pin base. Squeeze it and push it through the shroud. Now you can unclip the horn lead from the bike to make it easier to cut and crimp new connectors on. (Sorry, no pics of that).
Once you have your wiring run from your auxiliary fuse box (you DO have a fuse box, right?)
Run it to your 40A relay. I put the relay right on top of the fan shroud and secured with zip ties:
Don't forget to reattach the radiator bolts, then you are ready to button everything up. It's pretty much simply the reverse of how you took everything off from here. Hope this helps!