The reason the relay is needed is the trailer has four pins, one for the ground, one for the tail, one for the left brake/signal and one for the right brake/signal.

Problem is that the Xterra has an amber signal and a red brake, so they are on seperate wires. So the relay makes it that the signal can keep on working while the brake is pressed. It also makes sure you don't overload the signal relay.

It's also possible to buy a universal power module with relay that you need to hard wire to the battery and into each of the vehicle wires for tail, brake and signal. But they are a pain in the ass and in the same price range as the vehicle specific kit, so why bother. I used a universal kit for my Civic and I would never go through that trouble again if I can get a the vehicle specific module.

And don't forget the most important reason for the relays: To protect your vehicle from bad trailer wiring. This is a common problem and the relays stop faults in the trailer wiring from damaging your vehicle lighting circuits and blowing fuses and such. It is absolutely worthwhile to use that protection, I would never go without.

Really for $50 the factory setup is a fine deal. The included instructions are easy to follow and take you through each step. You get all the relays (I think there were four of them), all the wiring with easy plug and play setup, and a nice robust 7-way socket for the trailer connection. Adaptors are readily available to convert that to 5-way or 4-flat setups. It works like a charm and is VERY easy to install. Plus then you have the same setup as if you'd gotten the car new with the tow package.