The Ford V6 coil Pack has 4 wires connected to it,
one is 12volts that comes on with the key
the other 3 are Grounds controlled by the computer
Google: ranger 4.0l coil pack image
You would use an inline fuse and a switch rated above the fuse rating.
You may want to put an automatic relay on that type of setup, when your turn off the key/engine the relay that passes power to the coil OPENS.
When you turn the key back on the relay stays OPEN so no power to coil.
This is an automatic cutoff, you don't have to remember to turn off a switch, and you will forget, for sure, lol, and that's the night it will be stolen.
To CLOSE the relay you would use a second relay which could be activated, with key ON, by a hidden switch or by turning on and off parking lights or left turn signal or .......any full time or momentary 12v signal would send constant 12v to Coil relay so coil would have power again until key is turned off.
Second drawing here has that wiring, "Momentary to Constant Output":
http://www.the12volt.com/relays/page5.asp#ctm
This page has the pin numbering for automotive relays:
http://www.12voltplanet.co.uk/relay-guide.html
30amp relay with connector is about $8
There is also an inertia switch in the passenger side kick panel, the fuel pump power passes thru this switch, since it is inside the passenger compartment already it would be easy to put a switch or relays on this.