[RMcNeal]

Mine actually do light up when on.
I was originally told that they would only light when the pad is actively heating. I have since found that to be untrue. The pads have a thermostat that turns the heating element on or off based on a preset temp. The light is on any time the switch is on.
If you have a clamp on meter, you can test the heating element based on the amp draw to see if they are actually heating. I can't remember the temp setting but it has to be pretty cold before the element activates.