I mounted mine up under the seat, above the rear hugger. Bought some LED license plate bolts (LED's are in the bolt head) and cut two holes in the tray under the seat. Put the LED bolts in (through the license plate, of course) and wired them directly to the connector on the rear license plate light. Cost me a whopping $15 and took me no more than an hour.
They are very bright, and even 10 feet behind the bike you can clearly see the plate. But, then again, I have a low mount exhaust, so I don't have two exhaust cans that can get in the way.
Look at my pics in my sig, you can see where I put it. Those pics are old and don't show the LED lights, but it's in the same spot. Click on the "Full Size" link at the bottom to get a good look; you can zoom in on the pictures.
2009 Aprilia Tuono - Ginger
2001 XR650R BRP (Big Red Pig)
2006 Honda 599 - Ex wrecked it :-D
2007 Honda CB900F (sold)
2006 Honda VTX 1300C (sold)