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It is common for the thermoswitch to fail It is unfortunate that the evaporator will have to be removed to replace this switch.
This is a transistor that the base is pulled low by the temperature of the evaporator core and that allows the PCM input to be connected to the 12 volts of the control head. This is a very weak signal so it can be pulled down by a faulty thermoswitch. I have had success cutting and splicing a new head on the thermoswitch so you would not have to replace the thermister just install a new connector/head
The thermister is about $75 at Hyundai and there is a TSB about relocating the sensing bulb if you do replace the entire switch
Here is the wiring diagram for this part of the system, you have done the testing that is possible
2003 Jetta left rear window blue plastic part of the