The wife unit and I always wanted the reverse lights to come on when we open the trunk. It has made a huge difference in ease of unloading shopping carts and walking back and forth up our icy driveway. Super highly recommended to anybody who wants to easily see what they're doin when it's dark outside. It also has about 900 Lumens (equal to 80watts) of LED Bulbs so I can really see what I'm doin back there...
I thought about doin it for along time and finally made it happen when I added a 3rd set of HID's and my wiring box was out and I was in the mood. It took a little extra time than I thought because the trunk light is a ground switch circuit and the backup lights are a normal positive switch circuit so I had to add a relay to make it work. The random leftover relay I grabbed first fit perfectly in that body seam and is held permanently in place with a squirt of Right Stuff gasket maker I know I should have used a color consistent wire but it's my car and blue works just fine, I have lots of it and it'll never be seen hahaha.
It's always a bad idea to cut the vehicles wires so I try and do a good job of off tapping of them to not have any future problems. Twist into, solder, liquid black tape and heat shrink tube.
The green wire at the tail light plug is the backup light wire. Relay pin # 87
The tan wire at the trunk light is the battery positive wire. Relay pin # 30 & 85
The orange wire is the switched ground wire for the trunk light. Relay pin # 86