To understand what I'm talking about, here's the deal. Your heater blower motor takes 12 volts. The way pretty much all American cars are wired, it takes that 12 volts straight from the fuse box, through a relay controlled by the "Hi" setting on the air conditioner/heater fan control. If you want the motor to run slower, you have to feed it less voltage. The way that American cars do that is to have a resistor network, where each resistor gives you a progressively lower voltage, and use the heater control switch to switch which pin of the resistor network gets sent to the blower motor. The higher the resistance, the lower the voltage, and the lower the fan.
Checking out my handy dandy factory service manual, I decided to pull the resistor out and test it. The problem was, where is the blasted thing? Finally after sufficient digging around I found where it is: mounted right above the passenger side footwell light. So I unbolted the light and unplugged it and put it aside, unplugged a bunch of wires under there and moved them aside, and there we were, two 8mm bolts holding this thing in so that it sticks into the airflow where it can get cooled down while the heater is operating. So I removed it and it turns out to be a flat circuit board with resistive traces on it. I take my ohm meter, and yeppers, the connection between the last two pins (the smallest one is the low speed pin) is gone, and a glance at the circuit board shows that the trace apparently exploded where the connector's pin attaches to it, probably due to a manufacturing defect that allowed moisture to get into the circuit board.
A quick trip to my local Jeep dealer and $25 later, and I have a new resistor, and have all fan speeds going again. And yeah, this was probably caused by a manufacturing defect of this Hecho en Mexico hunk of junk, but my OEM warranty ran out at 36,000 miles, so fat chance of getting reimbursed. WTF, it's just $25. So anyhow, if you ever run into this with your own car, it might be as simple as that: a $25 resistor panel might be blown. It's a cheap and easy first thing to check anyhow, and it's not as if they're (usually) expensive...
-- Badtux the Now-Warm Penguin