Heater blower Low/Medium setting

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Could be the resistor, the switch or the wiring.  

Use a multimeter set to Ohms and measure the resistance from any of the three resistor triple plug terminals to the single terminal for the blower motor. You should have one with near zero resistance (high speed), high resistance (low speed) and one in between (medium speed). If any of the terminals return an infinite resistance, then the assembly is bad. If the resistor tests good, then test the plug for voltage. At least one of the terminals will have 12v in any of the three switch positions. If you have no voltage at any terminal in the low and medium positions, then it's probably a switch issue. You can carefully disassemble the switch and inspect. Often the contacts get worn or burned. You can add solder to build them up to work like new. 

 
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