I used the extension USB cable that came with the GXM to install it under the passenger seat on my '06. From the Zumo mount route the cable down and along the tank, then under the seats. I removed the silver trim piece, but didn't bother to lift the tank. Took 10 minutes, tops. It's out of sight, out the weather, works great.
It depends on your style...The antenna does not care about the seat or the leather, or the plastic of the fairing or a bag with clothes and other relatively transparent items. It does not like dense objects like metal or your body.
In my experiments, in my truck, in the garage with the doors closed and the antenna on the dash I get full signal, both on the Zumo and on the trucks XM (stock GM) I was able to maintain full signal in the glove box, center console, on the floorboards etc. As soon as I hid it under the seat, signal went to little or none. The seats have power and electric heat and are very dense signal-wise. I was even able to duplicate signal shading from the cute little metal plate that came with the unit by mounting it and turning it over. Note that the antenna did not seem to care about the thinner metal roof or the truck, sunroof or not.
As for your body, I could shut the signal down by placing the antenna on the north side of my body (satellite is in the southern sky) or even putting it in my armpit. I would be very surprised if you could get a reliable signal for the rear seat location with a passenger onboard, hopefully we can get Jimmy to confirm.
I still haven't figured out where to mount mine but I really would like it to be but I want it to be permanent and out of site. I'd also only liek to do it once.