It's just above your right shin when your foot's on the gas pedal (likely taped to another harness).
Here's a pic I found.
http://www.clubfrontier.org/forums/f23/05-08-frontier-tow-relay-install-hitch-pics-27530/#post548960
Here's a pic I found.
http://www.clubfrontier.org/forums/f23/05-08-frontier-tow-relay-install-hitch-pics-27530/#post548960