an electrical nightmare , please help , 1972 dart

Yeh the dimmer is a SPDT switch, so one terminal receives power from the light switch, the second feeds off to the dash bright indicator, and out the bulkhead to the high beams, and the third feeds out to the low beams. You are correct==2 at a time.

IT MAY BE that the lamps are not GROUNDING, which would feed "evident" power back on the unused line. The way to find out is to turn on the lamps and measure at all three lamp terminals. Only one lamp terminal should be hot at one time. Unplug both lamps and see if the "3rd wire" goes cold