I installed one in the back next to the parcel shelf.
It's not key switched (-always ON -that's what i wanted) BUT I am very careful not to leave anything in to drain the batter.
That way i can plug something in the trunk(leave a phone to charge out of site, or a 12v powered cooler box, or the back passengers can recharge their phones.
Got power from inside the car at the fuse box, used one that was not connected.
Installed an additional inline 20/30 amp fuse. The ground i hooked up to a bolt on the body after i stripped away the paint.
Used parts from Fiat Stilo the connector for the body computer and the 12v socket.
In the back of the center console would be much easier since you could just take 2 wires from the original 12v socket, as long as you are going to be careful how much power you are going to be using on both sockets.
Get the original Stilo 12v soket as it's good quality.