This looks like a nice Mod
I think I'll try it soon.
I think the ideal hookup would be to run some relays so that the lights will be on if either the backup or cargo lights are running. Then add an over-ride switch so they can be turned off. This way you can have it setup that whenever you hit the un-lock button on your keys the lights will come on with the cargo light.
... but I will need to do some research to get this to work