Wiring , 2005 Dodge Ram

I have a 2005 Dodge Ram 1500 Quad Cab 4 X 4 with the 5.7 Liter Hemi and 5-speed automatic trans. It came equipped with the factory installed tow package. I'm trying to hook up additional reverse lights and I would like to splice them into the harness which leads to the seven pin connector for my trailer lights. I have no concrete idea as to which wire is for what and would really like to know for sure before I use the "Trial and Result" meathod for wiring. All I neend to know is which wire in the harness is for the accessory/reverse lights in this harness for this 7-pin hook-up. I can improvise a ground with a self threading screw in a bumper support.

Thanks!

Thanks for your help and quick response. 

Well, I gave up on hopes for an easy answer and took out the taillight, got my test light, and put momma in the front seat and told her to put the truck in reverse and turn on the ignition  WITHOUT STARTING THE ENGINE!

I made my own ground by drilling a self-tapping machine screw into the rear bumper mount and attaching my ground wire to that. The wire I selected is a white wire with a light green stripe (looks more like a green stripe with a yellow border). I gave up hopes on splicing into the trailer light harness and spliced into the tailight harness intead. This required much less wiring and will hopefully hold up better when ice accumulations build up on them this coming winter. 

Just thought I'd throw this out there on the forum in case anyone else is interested in adding reverse lights to their '05 Ram.