In the low heat setting, current flows through the bottom cushion and seat back elements in series. So essentially, each element gets about 6 volts across it. In the high setting, they are in parallel, so each element gets the full +12V. That's all the relays do.I am also trying to understand how the high and low heat settings work. Does the high setting cause current to go through both output circuits?
When I did the wiring for mine I ran the power for the 2 seats through another relay, powered by a fused unswitched circuit. The coil on that relay was energized by a switched +12V circuit. That way, the ignition switch did not get the full current load from the heaters.