Turn signals don’t work with key on.

I would start with the basics; test the charging system and confirm everything is working to spec. If the charging system is not working correctly; you can get all kinds of odd behaviors from various electrical powered aspects of the car.

Here is a good testing video to review:


The alternator charge harness, is a classic problem; causing charging problems and melted fuse boxes.


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What brand alternator and battery; did you buy and install?

What is the vw you are working with (year, model, engine and trans type, etc)?


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Here is a video, going over parasitic draw testing:


If you narrowed down the heated mirror to be the issue; you could unplug it and see if the draw and dying battery are resolved?


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Often bizarro electric behavior is caused by a bad ground connection or a short in an unrelated circuit. The juice will flow backwards through other circuits to get to ground. I had a car where the diode shorted out and the gauges went nuts. On another car the tail light plug melted and cross wired: hit the brakes and the interior lights turned on. With newer cars, everything is digitally controlled and low voltage will surely affect it. A running car is typically at 14.4 volts.

The alternator bolts to the engine and that is the ground for the charging system. I'd check that. Clean the matching surfaces well (a bit of sand paper), and tighten the bolts. There is typically a ground strap from the engine/tranny to the car body that could be messed up.