fuse box

I'm working on a "smart-home" "IoT" project, and I need to plug a 12v to 3v Voltage Reducer into something that's always on. It's powering something that is normally powered by a coin battery, so I'm not worried about draining the 12v batter in the Volt. Just want some guidance as to where to...

I want to install a ham radio and/or a scanner into my 2016 Chevy Volt and was going to use a Micro2 fuse tap, but what slot number should I use on the rear fuse box? I'd like for it to power off when the car is turned off. Thanks