I have a 2002 Xterra and the blower for heat and a/c no longer works for settings 1,2 & 3, but does work on 4, the highest setting. Unfortunately, setting 4 blasts out so much air its like sitting in a frig when i have a/c on or a furnance when I have the heat on. Someone told me there is a resistor of some sort that needs to be replaced. Anyone know if it's an easy part to change?

Blower motor Resistor.. that is your problem. Easy to fix and replace

http://mywebpages.comcast.net/xterra/tech/BlowerResistor.jpg

$25 part, easy to replace.