The good panels either vary the width of the pulse (@12 volts) or the frequency for which a narrow pulse repeats thus varying what is referred to as the "duty cycle" and therefore the "effective heating level". The wider the pulse or the more often it occurs the hotter the plug will heat. A version of this is used in commercial airliners to allow dimming of fluorescent lighting.

You would need an oscilloscope to monitor the panel properly. This would raise the price of the panel though.