Use the search for "Heator Motor Resistor". Should bring up a thread I started awahile ago that will tell you why you only have the high speed and also show a diagram from Chevy Mgr. where the resistor is located. I got one from NAPA for around $40 if I remember correctly and only takes a couple minutes to put in.
Have you noticed any water leaking in on the Pass side? Probably caused your problem by shorting out the resistor and will need to be repaired or you will have the same problem again.