There is a high speed relay. It works like this: the voltage is applied from the junction block through a fuse on the firewall for high current. (Red wire I think) Then it goes to the blower relay. When the switch is put on high, B+ from the ignition fuse runs through the (most likely orange) wire through the relay coil to ground. This closes the circuit from the Red Junction block wire to the purple wire that goes to the motor running full current (high speed). In Medium Ign fused B+ drops a portion of it's voltage through the blower motor resistor sending lower current to the motor. In Low it runs through the medium and low resistor knocking the voltage down even lower resulting in the slowest speed. Basically you probably have a bad connection at the switch. Check your connections there first if everything preliminary checks ok
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