Recommend a horn

[brimo]

depends on how ambitious you want to be about it IMHO. On my old Monster, I had it mounted under the triple tree and ran the wires upward to the stack of headlight wires before going back to the bike. Was it plug and play?...no, not exactly, but it was effective when done...

No doubt the nautilus is effective and can be fitted to any bike, it's more a case of fitting it so it doesn't stick out like a sore thumb. Being an electrical/electronics type it's no drama for me, however the original post did say "without having to relocate or rewire".

[Mike Qube]

I have the Piaa horn. It's loud and is plug and play. It took me about 3 minutes to install.

[ungeheuer]

No doubt the nautilus is effective and can be fitted to any bike, it's more a case of fitting it so it doesn't stick out like a sore thumb. Being an electrical/electronics type it's no drama for me....

+1  I managed to hide a Nautilus on my M696 >>  http://www.ducatimonsterforum.org/index.php?topic=21649.0  <<  And I now have one on my M1100 (where the airbox no longer is).   Gotta be heard but not seen coz they're as ugly as they are loud.  I love 'em 

But....

...the original post did say "without having to relocate or rewire".

I'd be going with the PIAA for a plug-n-play option.

[Autolycus]

  Steibel Nautilus - Truck type...! It's big, it's very loud and I've found the best place to put it on my M1000Sie...
...Unclip the plastic horn section from your compressor body - there's a little plastic clip on the top that just bends out of the way - then prise it gently up & off. Now you have a compact compressor in one hand - which I'll mount with a relay either under the tank or down on the left side - and the trumpet section in the other. This clips straight on to the upper left fork, with a neat cable-tie or two to keep it there, and all you need to do is find an old air-hose barbed fitting, screw/glue that into the air inlet hole of the horn section, then connect that to your compressor outlet spigot with some thick-walled polyethylene tubing and a couple of small (fuel line), hose clamps to hold everything tight.
  The relay wiring is simple - mine came with a standard 4-pin relay, but you can pick those up in any auto parts store. Standard Relay Wiring codes are as follows:
Relay pin # 30: 12v (fused) supply from any 'Ignition on' circuit. Fuse should be 15 Amp rating for this horn.
Relay pin # 85: Earth/ground wire (from original horn wiring).
Relay Pin # 86: Horn Switching wire (from original horn wiring).
Relay Pin # 87: Relay output to Positive terminal on new horn Compressor base.
  There is one more pin, which is the earth/ground return from the Negative terminal on the new horn Compressor base - which, strangely enough, goes to a GOOD ground - preferably straight onto a 'bare metal' connection.
  I havn't actually mounted my compressor yet, but will publish pics when it's all connected up and Parping at sleeping taxi drivers - I'll also see what happens if I link the original earth/ground wire from the old horn, relay pin 85 and the negative terminal on the compressor, all together. 
  Finally, when it's all connected up & bolted down, do check that you still have full steering travel - then you can remove the old horn and give it away or keep as a spare. 

[Tdps2r]

Hey Autolycus, got any pics?