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The problem is those built in thermostats that turn on a ~40F. The circuit needs to be on and if it’s not cold enough to turn it on measurements won’t help.
I’m suspecting that the singe wire behind the switch is the main 12 volt from the battery. When you turn on the switch current flows thru 3 wire to the 4 pads. As you said 2 pads on fresh water tank and thus 1 each to grey and black. Right?
So how can you determine if the circuit is active? Hmmm. A feedback wire downstream from the thermostat would send power to a LED light by the switch. Runnng those wires would be a PITA. A button thermistor applied to the surface or between the pad and tank would give a constant reading of the temp. If the temp reading was higher than ambient temp then the pads would be heating only while in the narrow temp rang tho’. But how to run the wire without a lot of work or having to remove the underbelly covering.
I need to think on this some more. Any thoughts so far?
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