Okay, Got it done. I tapped the cargo light wire (noted in previous post) in the drivers door under the doorsill. used a quicktap or whatever its called and tapped in a fuse holder with a 2 amp fuse. Then i ran an 18ga wire through the bushing that the parking brake cable goes through, and zip tied it up to the harness that goes to the back of truck. There I put a relay, with a diode before the relay and a diode across the relay coil. I cut the B+ wire in the add a harness (7 pin for gooseneck) and placed the relay in there (the relay switches the B+ to both 7 pin plugs now). I had to go by pins instead of wire colors because the add a harness used different colors than expected. This way the only place i've touched factory wiring is at the quicktap at the drivers door.
Heres the tap at the drivers door. The buttsplice (top left) is connecting the fuse holder to the 18ga wire to the back of the truck.
Heres the relay at the back. The white wire with yellow connector provides ground for the relay coil.
I also added another relay on the trailer that is switched with the running lights, so now, when we're using the trailer at night, as long as the running lights are on, we just hit the cargo light button and the worklights on the trailer come on.