Otherwise, how do variable temperature seat heaters work? Do they lower the resistance to increase the temperature?
Nearly all variable heaters, including stoves, room heaters etc., are controlled by turning them on an off fast enough that the heat appears steady.
With a seat heater, to be fast enough it would need to be more often than every 5 seconds or so. It might be much faster.
There may be temperature control, with a temperature sensor in the seat somewhere. Then the heating is on full until the temperature is at the correct temperature. Then the various settings are the different temperatures.
Without temperature control, the settings would vary the power by varying how much of the time the power is on. It might be something like where low is on for 2 seconds and off for 3, medium is on for 3.5 seconds and off for 1.5, and full is on all the time. (Those figures are guesses, just it illustrate the method)
If the control is much faster, maybe thousands of times a second, if you measure the voltage across the heater it will appear to be less than 12 V. So if the power is on for 0.001 second and off for 0.001 second, with a 12 V supply, that would show as 6 V on a meter.