There are 3 wires at the switch. One power in, one power out and one ground. 2 seperate circuits.
Light gets lit from the common power in wire, switched through the switch and the ground wire.
Heater is powered by the common power in wire too. The power goes through the switch when turned on passing voltage down the 3rd wire going to the heater pad. The heater pad is grounded by a different wire not realted to the switch.
So 2 seperate circuits. One circuit to light the light and one circuit to provide positive to the heater pad.
The ground wire at the switch is only used for the light. The ground wire at the switch does not have any bearing on the heater circuit at all.
Clear as Mud?
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400,000KM and still ticking