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Well, I just looked back and realized I may be completely wrong... Dunno about the 300SD.
I'm referring to the push button unit (if your car has one).
The relays for the fan and whatnot are in there, and the connections can fail.
The one thing you can try to confirm this is try turning on the blower using defrost mode, and then switch to the mode you want.
There are two blower relays, one for the right two modes, and one for the left two.
However, once the blower is engaged with either relay, the unit keeps it engaged with that relay until you turn the ACC off.
That way you can see that one of the relays has problems by trying the other one (the economy/normal mode one typically has trouble while the other one is fine).
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