Hello,
Good idea, but I took your step little different. I have the puddle light and I tapped into that.
This way, when vehicle is in "run" position, puddle lights and the cargo lights will be completely off (as would the cargo light switch would too)
I also have a overhead canopy light, so it HAS to be off when backing up.
I have attached the diagram, REMEMBER, the orientation of the diagram is with the plug out, and the plug end facing you with clip fastener facing up!
**** TIP****
I have had trouble keeping the leads of the diode in the wire socket, I end up sanding it sharp so I can get it in deeper next to the exposed area of the wire. IT ALSO HELPS if you wiggle the wire to the opposite direction it is leaning on to and insert the leads in that side. Wiggle the wire to the other side allows it to push back and keep the leads of the diode in the pressured position to maintain a solid connection.