Well, the ground will be common, all you need to do is take a cable with +12V from a fused spur, i'd suggest ignition live unless you want to power the switcher when the key is in the "off" position, you can get this from your radio harness, don't forget one will be permanent live for backup as well as ignition, so chose the correct one, a 0V / Ground (whichever terminology you wish to use), a "switch" wire (to one side of the switch) and a +5V from the regulator to put to the other side of the switch. The pull down can be inside the box itself and can be hidden away.
You can if you want to be really clever, put two led's near the switch to indicate which one is "active" not necessary, but a nice touch, you can wire the LED's back to back so only one comes on at any one time.