While the resistor pack is a common issue and very likely, there are other (cheaper, easier) possibilities to check first. The simplest is that the hose connecting the blower housing to the distribution plenum is loose (mine has done that). The blower motor also has all the current pass through the control panel knob, so see if there are any visible signs (burned wire, loose wire, etc). You might also be able check the resistance to ground here for the three outputs (high speed has no resistance except for the motor, and medium and low speed are increasing resistance). Since you have the motor inline, this will only work if the motor parked in just the right way, so it may read open for all three whether or not there is a problem, but if you get reasonable resistances then that rules out the resistor pack.
"Lots Of Trouble; Usually Serious"