How is the control head? Try to isolate the fuses and relays associated with the climate control blower motor. You could possible have a wiring problem, sticking relay or faulty control head.

I have a 87 420SEL at one point. One day the blower motor would just come on by itself. I would even Press the "O" for off and nothing would happen. Problem ended up being a faulty control head.

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2001 E55

2001 E320 Sedan

2001 Subaru Legacy

2006 SLK280

2016 Subaru Outback 3.6