The first thing to check is the resistor assembly. This is mounted on the evaporator "suitcase" under the hood. You'll see a number of multi colored wires with a plug going into it. Unplug the connector and inspect for corrosion or damage. Then remove the 2 hex head screws and pull out the resistor assembly. They are wire wound resistors that resemble springs once pulled out of the case. Many times these corrode or go open from age. The resistors drop the voltage for the blower speeds--every speed but high.