You have to look for voltage with the switch in the OFF position (open circuit) and when you turn the switch ON (closed circuit) the voltage should go to 0 volts. To really tell if the heater is working, you will need to measure the amps going thru the wire. You can put a multimeter, set to amps, in series with the switch and one wire. However, if the current draw is more than 10 amps, you will blow the fuse in the multimeter, but you will know there was current flowing.
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