Overcomplicating things. Easy button, 4 wires from add on lights, to 4 flat pin, done. If you want to ground, need to make contact with bare metal on truck, also super easy, but no reason to run a separate wire to the truck frame. Heck, the hitch where you hooked might not even have a good ground if the frame and hitch, both painted or coated, didn't rub off enough paint anywhere to make contact.
Avoid extra happy homeowner wiring, like this, I can show you the ball of "electrical improvement projects" wires and plugs I gutted out of the 1986 pickup I'm restoring. None of it worked anymore. Back to just factory wiring, all the lights work fine and I can re-wire a trailer light plug the right way.
