The most common cause for this issue is the ground connection in the 74/602 and 74/603 connectors at the rear end of the roof.
You can easily figure that out by temporarily connect a separate ground cable between the car floor (the are ground studs easily available) and the right side of the rear window heating connections. There's a 6.35 mm terminal available at the window. You have to remove the inside trim on the rear door to access that. Temporarily tuck the cable to the side and see if everything works fine for a week. If it does, install the cable neatly.
That's what I had to do.
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