Sorry, I thought I deleted post. I did find the answer in the search. Thank youIt's a light blue wire. Two place where you can find it: in the relay box in the engine compartment (can't go wrong using that); and in the trailer wiring bundle behind the passenger kick panel - but I think that's only good if you have a manual.
Remember to search the forum for info! This has been covered recently.
http://www.clubfrontier.org/forums/f24/2005-up-reverse-wire-d3-install-complete-21568/