Where exactly are you trying to tag this into? If its the tail light (Left) the wires are WT/DG(Dark Green) This is the rear turn lamp driver. WT/LG(Light Green) This is the park lamp feed. And BK This is the ground wire.
This website is where i get my wiring diagrams from. you might want to see if you can go into the cargo lights that are into the third brake light rather than the running lights on the back. you might also consider going to the front headlamps where the front running lights are.