Yep,
Already did this. I added led lights under the bed rails so I could see what's in there at night under my tonnue cover. I tied into the bed light switch. You can use the same method to run any other lights you want back there.
Its a simple process, but you will need to use a relay to do it as the BCM looks for a specific voltage on that circuit. Adding more current draw on that circuit causes the bed lights to shut off after 3 seconds.
So I tapped into the bed light as the relay trigger. (top right of photo with the T-Tap) and pulled power from the trailer hitch circuit to run the additional LED lights in the bed. Installed the fused relay right to the body behind the tail lights (bottom of photo) so it would stay dry.
Works well with no issues in 6000 miles of running this truck.