The antenna adapter was the key! That's got to be wired up before you can get power to the unit.
My original plan was to rig the whole thing up before putting it in the dash at all, but I got anxious and wanted to see it light up, so as soon as I had the wiring harness ready, I plugged it in and got nothing. That sent me on a wild goose chase, trying to fix something that wasn't broken.
As you can see from the pics, the head unit and XM radio are installed and looking great. In a 2012 sedan, this install requires NO dash kit - just the factory brackets.
Anyway... thanks for the help on here! Our Versa is my wife's car and she loves it. I'm encouraging her to join this group, so you should see her on here soon.
By the way... I had originally gone to Best Buy to get this installed, and it was a fiasco. Read on if you're interested.... My wife trusts Best Buy and really wanted them to install it. $420 for the hardware and labor! Grudgingly, I agreed to it and we paid it. When we showed up for our appointment to drop the car off, no one was there to inspect it. We had to leave the car and come back in the morning for inspection before they could start the work. Our appointment was for 9AM - we even got a confirmation email about that. When we showed up then, we found the place empty. The store didn't open until 10AM, and the installation department didn't open until 11! When they finally opened, they said we needed the dash kit and that the salesman should have told us that. They don't even carry that dash kit and said we had to find it somewhere else and make a new appointment. I told them I wanted my $420 back and I wanted the names of everyone involved in this fiasco, and that I was calling the corporate offices to report this craziness. I did report it, but all they did was apologize. I bought the exact same hardware on Amazon for 70 bucks and installed it all myself.