I just completed this successfully on a 2017 Silverado 2500 High Country.
Used the picture from post #4 - the wire colors were different but same locations, I took Coax cable and left ~3/4 inch of insulation and had 3/4" of bare copper wire on each side. Bent the wire 90 degrees and used wire cutters to put a slight point on the end. Pushed into the locations on the pink and brown plugs and worked just fine.
1. If you have a backup camera, turning the cargo lights on via the switch, also changes your Infotainment screen to show the backup camera
2. If your headlights are set to auto, and it is night time - when you get out of your car and lock it, the led lights in the mirror stay on until the delay for the headlights to turn off.
3. If you unlock you car via the key fob, the led lights will come on, locking it again turns them back off
4. Putting the car in reverse will illuminate the cargo lamp button - even though your dash info will say cargo lamp is off
5. If you use the cargo lamp button - it gets confusing as to if it is on or off, its best to not trust the button, go by the info messages in the dash
I suspect if you use the diode that some of the "weirdness" listed above goes away, but I do not have a diode to test it with.