Installation: (all wiring is 14 gauge, and I soldered all connections)
As with the P-D, the most difficult part was deciding where to mount them. In the end, I decided to mount them inside the fairing, on either side of the headlight. There is not much room inside the fairing, so it was not easy to find a place to mount two rather large horns. I doubled up the mounting straps for strength, then bent them to a 90 degree angle about 1 inch from the end which mounts to the bike. I fastened the strap to the horn. I then removed the bolts which hold on the oil cooler (one at a time!) and placed the mounting strap under the frame attachment point. I bolted the supplied relay to a hole on top of the wiring junction box in the left upper fairing.
To wire them up, I removed the stock horn, and noted that the green/brown wire went to +12 V when the horn button was depressed. I then ran the stock horn wires to the supplied relay. (The bike already has a relay for the horn, but the wiring was simpler if I just had the BMW relay drive my relay. I don't know how tough the BMW relay is, either.)
I then pulled out the battery and attached a wire to the hot terminal, ran it through a radio shack inline automotive fuse holder with a 15A fuse, and ran that wire alongside the main wiring harness to the new relay.
I believe that one difference between these horns and the Fiamms is that they ground through the mounting strap. On the P-D, I ran a separate wire to ground the horns, since the crash bars were not well grounded. I did not have to do this on the R1100RS, since the mounting point seemed to be well grounded.
That's all there was to it. It took me a few hours, since I took off the fuel tank. This is NOT necessary, and I strongly recommend against it.
p.s. yes, they are loud!