I have this done to both of my Dodges, My Ram as of today, and my Dak..
Only person I know that has this done so far, until more of you do it!
Great mod for guys who don't use cargo lights, or can't use them because of a tonneau cover in the way.
Video of the Dak: (Didn't have load equalizers when this was taken, they don't blink so fast any more)

Needed:
Red LED or Regular 194 Bulbs
(2) 15' 16ish Gauge Wires for Turn Signals
5' Wire for Ground
(3) Butt Connectors (not that kind wierdos) or (2) Male and (2) Female Connectors
(2) Quick Splices
(2) New 194 Sockets if you don't want to cut your wires.
Electric Tape
Zip Ties
Tools:
Phillips Head Screw Driver
Plyers
Socket to Remove Seat
Extention
Socket Wrench
Wire Strippers
Crimpers
Power Tester
How to:
1) Remove the driverside rear seat
2) Remove the third brake light from the truck.
3) Starting at the rear of the truck, run the two 15' wires from the back of the frame where the trailer wires are to the back of the cab.
Use Zip Ties, or Tape to bunch up with exisint wires.
Hint: If you can run them in the same loom as the other wires, it will look a lot better.
4) Run wires up the back of the cab into the vent.
Hint: Use another wire to fish the wire up. Drop the wire down to grab it.
5) Attach the 3rd wire to a ground, I would recommend using one of the bolts on the back seat loop things.
6) Route the three wires up the side pillar, hiding them.
7) Remove the back hook, and run wires above the headliner, and then out the back of the hole for the third brake light. Put hook back in.
8) Remove the third brake light from the truck.
9) Go back to the back.
10) Use quick splices to splice into the two trailer harness lights for the Turn Signals
Yellow: Right Turn Signal
Green: Left Turn Signal
11) Cut the two wires from each cargo light sockets.
12) Matching colors, ground both bulbs together.
13) Test the left and right wires to figure out which are which and label.
14) Use male/female connectors or butt connectors to wire to coresponding socket.
15) Test to make sure they work correctly, and put seat and third brake light back!