I have a rear window defroster setup that has 2 leads going to the element. Recently the system has not been working. When the dash switch was in the on position the indicator light didn't come on as showing the system was working. I checked both element lead wires with a test light and found that only one lead has juice to it when the dash switch is on. I cleaned both leads and plugged them both back in and nothing. No indicator light on the dash switch. I unplugged the lead from the element that had no juice to it and turned the dash switch on and got an indicator light. WTF? Can the element function with just one lead plugged inand supplying power?

One lead will not show power since that is the ground, and by Ohm's law, the load on the circuit MUST consume all the voltage supplied to it.

I would look between the power distribution and the switch, it sounds like a bad connection causing a lot of resistance, and when the load is connected, it doesn't allow enough power through to run it and the indicator light. Without the load, though, there is enough power to run the light.