Carl --

I looked inside the sensor of my '83 (had to repair it) and found a series of coils. From the design, I'd guess they're bifilar-wound with a tiny reed switch inside. As long as the current was equal (and opposite) in each winding, the switch experiences a neutral magnetic field -- when one or the other coil loses current, the switch experiences a field and closes.

I do not know how the newer cars account for the third circuit.

If I were installing a third brake light on an older car and wanted to avoid this problem, I'd feed it from both both sides through diodes -- perhaps 8 amps each. The diodes would easily pass the current (with only .6 volt drop) but would prevent "cross feeding" each to the other side. Unless the diodes are very mismatched, they'd share the current. Of course, you'd need to orient with the correct polarity -- or the third light will receive no current. I'd guess you can buy suitable diodes at Radio Shack for a few dollars (50 volt, 8 amp, or so). Correct orientation is with the diode "arrow" pointing toward the 3rd light -- or the band closer to the 3rd light.