"However, looking under my dash, I noticed a multi-pin (sorry, didn't check if it's 12 or 16) connector that runs back in towards the head unit. Is this the XM pre-wired harness? I'm not talking about the mega-pin connector attached under the dash where the driver's knee would be (which I figure that's a connector for Nissan mechanics to do diagnosis)."
The only other thing under there would be the wiring for the brake controller (if so equipped). Mine has a sticker that clearly identifies it as for the brake controller. There are pictures of the XM NBUS cable in the installation instructions:
http://www.leeauto.com/parts/titansatellite.pdf
"Btw, got my micro-antenna wire ran just like bvstone1 suggested, that's a great way to do it. Has anyone that's done that had problem with water leaks though? I'm paranoid that I didn't seat the light back firmly and will get a deluge into my rear passenger seats."
I don't think you are going to see a "deluge". Maybe a little bit of water that will corrode the brake light connectors, though. You could always test it by spraying the hose on it for awhile and then taking a look inside. If you are really paranoid you could run a bead of GE Silicone II.