Blower motor resistor... available at most auto part stores, napa, oreilly's, auto zone, advance, car quest, Checker.
Or you can buy it at the dealership or online.
Just tell them you need a blower motor resistor for 200* Xterra. They should be able to help.
If you can successfully rotate a phillips screw driver, remove a electrical plug. You can do what needs to be done. Its a rather simple as well as common repair. In the 3 years I've owned my xterra I've replaced it twice. Put your head on the passenger floor board and look up under the dash. You'll see a connector with wires. What those wires plug into fails and needs to be replaced.