4. The ’67-’68 switch controls the ground of the resistor block through the wires shown on the switch for low, medium, and high. You can check each terminal for power to verify if the wiring is good. You can also disconnect the switch and check it with an ohm meter to verify that the switch works. As a final note, the switch grounds through the control panel and dash, so make sure you use a jumper lead to ground the panel when testing the switch or you won’t “see” any voltage with a test light. Our problem ended up being a bad blower motor.