How about making the cargo lights come on with the backup lights? An easy way to do it is to tap the backup lights hot wire and put a diode in series with it to the cargo lamps hot wire. When the backup lights come on so will the cargo lights, and the diode will prevent the backup lights from coming on when they're not supposed to. If you wanted the backup lights on when ever the cargo lights are on, you could eliminate the diode.
What do yall think?

 

I did this last summer, used a 6 amp diode, I wouldn't do it without a diode, can't see at night backing up with your dome light on!

 

Good point!