1997 B3500 ac/heater fan works on high only

[rvdude]

My blower fan switch has slow,medium and high settings. It works on high only. I removed the switch and found carbon build-up in it. I also noticed the leading edge of the plastic female connector has melted. Replaced after cleaning up but still same problem. Tried looking for the resistor to see if that was it but could not find it... HELP!!!

[alloro]

The resistor is under the hood, passenger side, mounted in the big black box.