Seeking positive terminal under dash

My wife doesn't drive her 2004 Prius regularly. Living in the desert, this means it's vulnerable to rats nesting under the hood and/or chewing wires. I've installed an ultrasonic beeper under the hood (which has given me decent protection on my Alfa Romeo), but this means we also need a small solar panel on the dashboard to compensate for the beeper's current draw. Looking under the dash, I don't see an obvious always-on positive terminal to connect the solar panel to. (There is an open fuse slot that's always on, but I'm leery of jamming a wire in there. And it would prevent putting the fuse cover back on.)

If I could get the whole white under-dash fuse box out and apart, I'm sure I could jury-rig a solution, but it's unclear which latches to undo to let me slide the box out.

Alternatively, if I could get a 2-conductor wire through the firewall, I could wire the panel to the hot terminal inside the under-hood fuse box, as I did with the beeper. But there's no obvious path through the firewall.

Any suggestions?