I've been busy more so than usual lately. Free time has been at a minimum to say the least.
Jamie, I checked the ETM, and it looks like fuse #5 would send power to the "+B RR FOG" input of the MCU if installed. - Don't know any more than that - I can't seem to locate any documentation on the MCU. :-x
However, if I was you, I would take a look-see at fuse #16, it's listed as 'Unused' and - if the fuse clips are there - you could plop a fuse in there and have a dedicated 'HOT AT ALL TIMES' output on connector G, pin #5.
If you're just wanting to tap into a circuit for your radio, at 4 watts it's not really gonna make a big deal which one you use. You're not talking that much current to begin with. If it was me, I would probably tap into fuse #7 (stereo amplifier) and use that one. (you are wanting a hot at all times source, right?)