2011 I just took one side rail off to replace a turn signal. The nuts are 12 mm, like Greg said. My big fat hands and arms didn't want to go in there, I got my son, with his skinny arms, to get the bolt started from the inside. I used a flashlight to look under the fender as well. I thought about jacking the bike up off the wheel, to see if that would give me a little more room, but son came to the rescue.
This is a racheting box end wrench. gearwrench is a brand name, but craftsman (sears) has them as well and I am sure others do by now.
Rounded off the bolt, maybe some vise grips.
I hate it when that happens.
But it usually does, at least to me.