The problem is dealing with A/C Heater Blower Motor Resistor it is not a switch issue because you know it turns on in high. Current flows through a resistor to the blower motor in order to give you your desired speed. When you switch it to High it does not go through a resistor and that is why it works in that setting only. Somewhere there is an open in your blower resistor. I checked at Auto Zone and they are going for around $20 and it is an easy fix.