so this is what you bought?
http://www.etrailer.com/pc-VWH~118399.htm?vehicle=2005%A0Nissan%A0Xterra%A0%20With%20Tow%20Package

i'd say your fine. The reason for the with tow package comment is i know for the first gen they didn't put the wiring in to use this plug in manual ones, just the autos. Since that plug relies on you having the factory wiring already they say factory tow package.