The dimmer switch, floor switch, doesn't have a ground. All it does is switch the current path to the different lights, high or low, and to the high beam indicator in the dash. Juice goes from the headlight switch into the dimmer switch then out to either the high or low beam wires to the lights and then the circuit grounds after the lights.

Real simple circuit, high beam wire out of the switch, green and black on a 68, to the outer and inner high beam lamps then to ground. Low beam wire, black and tan on a 68, out of the switch to the outer low beam lamps then to ground. The low beam wire goes to the left high-low pigtail and from there to the right high-low pigtail. The high beam wire junctions near the left light and from there one wire goes to the left high-low pigtail, one wire goes to the left high beam pigtail and the third wire goes to the right high-low pigtail. A wire goes from the right high-low pigtail to the left high pigtail.

Check that the wires and connectors at the high-low pigtails aren't somehow shorting from the high to low circuits, that would have all the lights on with either high or low selected.

Maybe your dimmer switch is faulty. Mine was corroded and the wires in the connector were touching so I had power to all the lights no matter what position the switch was in. You could take off the switch and test it for continuity from the power-in connector to the high and low connectors, and check that the switch isn't connecting both high and low to the power source at the same time. If the switch is good you have some wires crossed in the harness.