Haven't looked into this in a modern car but in the old days the heater works by switching on a valve to let radiator "coolent" flow into the heater assembly. There often is mechanical linkage from the heater control lever, but I suppose it could be electrical, that turns off/on this valve. You should be able to see radiator hoses (probably 3/4" or 5/8" in diamter) going through the firewall in the engine bay and you may be able to spot the valve.
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