I may have misunderstood
I thought you meant it blew high speed if you set the temp dial to the defrost detent, and were looking for a way to reduce blower speed. If you want a lower temp air blowing to the windshield, that should happen by pressing the directional control without changing the temp. I usually use the windshield/feet button, and as I sit here I can't recall ever using the windshield only (it was a very mild winter).
If I understand my manual correctly, the temp wheel governs temp plus switches blower to high speed when set to the full cold or full hot position if the blower is set to auto. Directional buttons determine outlets where airflow is directed. As for fan buttons, top is always high, auto is auto unless temp is at high or low detent, and low is always low.
It's about 90F here today, so I'll run through all my settings on the way home and see if the windshield only directional button changes temp in my car.
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)