The blower relay is sometimes called the heater relay. It is above the glove box, see the pics in unrelated thread
viewtopic.php?t=5078 The relay could go bad, but don't remember anyone here ever having that happen. Might be loose though, should be able to reach up and around that junction box and press the relay back down
Some folks here have had the actual blower motor wear out to where it becomes intermittent, sometimes going on or off after a good whack or sharp bump in the road.
And as ehoff said, could be the blower resistor. Classic sign of a bad resistor is fan only blows on highest speed, or perhaps only highest two speeds. But a bad resistor isn't intermittwnt, wither the low fan speeds work or they dont, not sometimes