Well, relatively simple at least.
I was tired of having to go into the truck to turn the cargo lights on and off, so I wanted to add a secondary switch in the bed. I searched and found multiple results about adding a secondary set of lights, which was not what I was looking for, so here's my solution.
I wanted to add a secondary switch to turn on/off the factory cargo lights, and retain the 10 minute auto-shut off. Through some searching I found that the switch in the truck works by pulsing a momentary ground to a pin on the BCM, which is pretty easy to replicate/extend.
Wire. I used some red/black 2-wire, I'll explain why later.
A small scrap of solid core wire.
2 waterproof push button momentary switches.
Here's what I did:
Soldered a small scrap of solid core wire to both leads to the red/black 2-wire. This made it easier to tap into the BCM.
Tapped into pin 18 of the blue plug on the BCM. There's a solid gray wire on that pin.
Ran the wire out of the cabin (using this method:)
and then down the underside, following a large bundle of wires and using some wire clips that GM/Chevy was nice enough to leave available for use.
Popped out one of the cargo tie down plugs, and ran the wires through. Here's why I used the red/black 2-wire - measure enough wire out to go back through the hole and then under the bed to the other side, then cut. Now we have enough wire to install a switch on both sides of the bed.
Drilled the plug to fit the push button momentary switch, and installed the switch in the plug.
Cut ONLY ONE of the wires at the drivers side hole, and wire it to one lead of the switch.
Connect the scrap of wire you just cut off (red in this case) to the other lead of the switch. Run both the red scrap and the black wire back through the hole, and reinstall the plug.
I didn't take any pictures of this part unfortunately, but run the red scrap of wire to any grounding point and secure.
Then run the black wire to a cargo tie down plug on the other side and repeat the steps to wire and ground it.
Then enjoy your 2 new readily accessible cargo light switches!
This activates the same as the cargo light switch in the truck, so any modifications you've made like the reverse lights on with the cargo lights will work just the same as if you pressed the switch in the truck, and retain the 10 minute auto shut off.
Any comments/questions/clarifications please let me know and I'll do my best to get back to you!