What you are looking for is ignition switched power. Since these scanners don't pull much current, this is a pretty simple thing to do if you have some very basic electrical skills.
From your radio power lead, connect the black/negative wire to ground. This could be body sheet metal, or a screw under the dashboard that is attached to the truck metal. You may have to look around, but usually under the center of the dash board you can find something. I have a 2009 F150 without the center console. You can pull the plastic off from under the center of the dash board by pulling the plastic pin on the driver side at the front of the plastic under the dash board, and the two pins on the passenger side. The plastic piece comes out by gently pulling the bottom out a bit to get it started, then the tabs at the top are released by pulling the top out. Under that cover you will see some aluminum supports for the dash. Use a self tapping screw to attach your ground there.
While you have that cover off, run your positive lead under the dash to either side of the truck. Under the sill plate, the narrow piece of plastic that would be under the door. Pull that piece straight up, it just snaps in place. Under that, on either side of the truck, you will find a wiring bundle. Inside the bundle will be a yellow wire that is slightly larger than most of the other ones. This is the power feed for the power windows. It's on a 30 amp fuse, so it will support your scanner just fine. It is also on the retained accessory power, so it will come on when you start the truck, and go off when you shut the truck off and either open a door, or the timer shuts it off after 10 minutes or so. This will let you listen to your scanner with the ignition off, but it will shut off on it's own.
You should install a 3 amp fuse as close to that tap in point as you can to protect your wiring. Be real careful about where you route it, keep it away from sharp metal, etc, anything that would damage it. To make it look good, use some of the black split loom tubing that you can find in the electrical area of most auto parts stores.
I use this same circuit in my truck through an external timer that feeds my two way radios. It allows the radios to stay on for a few hours after I shut the truck off, then shuts them down automatically.
There may very well be other places to find ignition switched power, but this is one of the easiest to get to. The issue with using the fuse taps is that it will spread the contacts apart inside the fuse box and make it difficult to get the circuit to work if you ever remove it. I would recommend NOT doing the fuse taps, unless you have no other choice.
