When my Prius is turned off, I wanted my refrigerator to stay powered on so I hardwired the new cigarette lighter outlet to the fuse box.
The Alpicool C20 Portable Refrigerator is a 12v/24v unit, taping into the Toyota Prius 12v system was a breeze!

The basics of the install involve using a fuse tap and a a cigarette lighter socket!
I used the driver door fuse slot as it has constant power when the car is turned off. Once the tap was wired up to the cigarette lighter socket, I used a ground under the drivers acceleration pedal.