Well, depending on how hidden you want to keep it, you might consider an additional power stud under the hood, with a wire straight from the battery. That way you could run all sorts of always-on accessories from it.
If you don't want to run the extra length of wire, or have additional wires under the hood, you could add a tap to the fuse block.
What year is this you're working on? Fuse panel in the glovebox, or on the floor?
I've used those plug-in taps with success before, but it's not my favorite thing to do. You could pull the fuse panel (or the whole glovebox?), pull it out and solder your power wire to the back of one of the lugs or solder joints on the back of the fuse panel without having to lay under the dash.
Since it's such a low power drain, and has a pretty non-critical function, tapping in is not such a bad thing. I just hate to see you mess with the main power feed to the ignition switch and risk messing up much more important things.
Perhaps the simplest constant power line to tap into would be the cigar lighter wire? Always on, more than enough capacity, and if it ever messes up, not a critical loss.
Easy to fix too.
Paul