I would start by removing the motor and fan and checking for any obvious problems. It's pretty obvious how to remove it, it just drops out from the bottom of the blower housing.
Your Xterra should be pretty similar to the D22 Pickup sold in Italy. You could try cross-referencing the US part numbers with a local dealer:
Blower motor w/ fan: 27220-8B400 (up to 9/02) or 27220-8B410 (9/02 on)
Fan only: 27230-2M100 (also used in the 11/94-12/99 Sentra/200SX B14U model)