I did this mod last night. Just to be clear, the three wires you want to splice are the brown/white, green/white and extreme lower right black/white wire in the harness. There will be two black/white wires in the harness. As stated above, the black/white wire you want to splice is the lower right one. That black/white wire, once spliced, will travel via new wire to the ground post on your 3.5mm jack. This will be the longer of the three posts if you bought the amazon jack. The other green/white and brown/white wires will be your left and right channels, so once spliced, run them via new wire to the other two available posts on your 3.5mm jack. It really doesn't matter which posts since the system is set up in a stereo format.
Finally, the last think I'd like to add is you can get by with 18ga speaker wire, but I found myself stripping it down (removing most of the threads at the connection) to make room for the connection. In other words, I found that it was a little too big for me on this job. If I were to do it again, I'd go somewhere else other than Lowe's (they only carry down to 18ga) and find some 22ga stuff to work with. I think this will make life much easier you guys who attempt to try this.