If you do the pedal test you get a code that will help u narrow it down. You may even get lots which will point to an issue with more than just the heater unit.
Always do the pedal test if your trying to fault find with anything electrical.
depress gas and brake pedal together fully to the floor
key and to the 3rd position. do not start the car.
watch the mileage display and your codes will flash up here. You will see code '000000' dont worry about that.
Post your code(s) here or google them to begin the fun part.
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