Hi, the issues is mostly in the control unit (head unit, where you can adjust temperature, fan speed and so on).
In german we call it Bedienteil.
If you switch the driver side temperature to warm, you will get a "click" noise at the end of the scala.
This click noise activates a switch which will take the 12 V of the wiring. It is meant to be an "emergency" function. If nothing works anymore, you can still get a heated car - just by switching of the 12V and getting a permanent open valve (without power the valve is always open)
Unfortunatelly that switch often gets broken and does not release anymore. Result is that you do never get power again.
You can measure if the switch has pass or not.
Another issues could be a rotten cable direct at the heating valve.
Markus
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