This is the steps I did to install bed lights in my 2012 F150 XLT, not sure if it matters or what other years work. I do know it is the same on a 2013 FX4! I have a total of $28.00 in this setup including shipping from Amazon. Please let me know how it worked out for you!
I ordered a LE Lampux 12v Flexible LED strip lights, waterproof, 300 units 3528 LEDS off of amazon along with HeroNeo 10 pcs Connector Adapter cable LED PCB strip 3528 to 3528 and then I purchased Black and Red 14 gauge wire at my local auto store. The installation was very quick and easy!
Make sure you wipe down the desired location of where you are going to install the light strips! I didn't think about it until they fell down then had to mess with purchasing 3m tape and redoing certain sections. Once you figure out how you are going to mount these lights and cut to the desired length, clean off the rubber protection cover on the end so these connectors will make good contact. this was the biggest pain because I just jumped in to it not being able to find out what to do! Once you got your connector installed to the LED strips in your corners, grab a 9v battery to test and make sure everything is connected correctly.
Assuming everything works its time to run your wires! Now when I first did this I thought I would just tap in to my trailer connection hook up. well it does work, great actually! lol, but the only problem is if your head lights are on, so is your bed lights! So, you need to run your hot(red) wire through your Driver side rear tail light section(easiest for me) down along my bed and frame rails to right under the driver side of my cap. I opened my rear door and removed the sill plate. once this is removed you will see a large wiring harness and a small plug about the size of a dime. remove the plug and push your hot wire through the center of this pulling it fairly tight so there is hardly no slack hanging under your truck. install the plug back in to the hole it came out of!
Next, you want to find the Grey wire with a purple stripe on it! This is the wire that gets power and turns your lights on when you open the doors and shuts power off when ya close them(sorry i'm really breaking this down Barney Style). Yo can get a cheap light tester to make sure this is your wire! It does work for 2012 and 2013, not 100% sure about other years.
Becareful when you install your sill plate, i bent the little metal clip and messed with it for awhile because I got excited and in a rush!
Here is a couple links to my Youtube.com account showing the harness and the bed lights working!
Lights in ACTION!