The black wire off the 3rd brake light should be your ground. The white the "hot"
Get yourself 2 of these:
Take out the tail light. Use the above pictured device to piggyback off the brake light.
--just remembered your truck only has 1 bulb for brake and turn correct? Hum, and prob does not have a 3rd brake light on the cab, just a cargo light.
In that case, you can use the above device to piggyback off of the brake switch on the brake pedal, run it back to the rear of the truck (make sure to run it safely) and then connect it to the white of the canopy light. The black can be grounded anywhere.
Does that help?