Top of dash vents are after A/C coil, but before Heater coil, so they always blow cold, coolest (under dash intake air, recirculation light lit), or outside air (presumably to keep the radio from getting too hot?).
In moderately cold temps I can only get the Heater Temp knob to modulate the temp if I close the top vents and push ALL the air through the heater coil. Otherwise in cool/cold air, we get hot, Hot, or HOT your choice...
Dash vents each close individually, but the knob goes from dash only at 9:00, to windshield at 12:00, to windshield/feet at 3:00, and to feet (only) at 6:00. No setting on the knob turns off the dash, EVER... If you like the air to come in through the top, then close the dash vents, set the knob to 9:00 and turn the blower up or speed up as the air is rammed into the system from outside pressure (unless you press the re-circ button).
This could be cross posted in our Sprinter Peeves section
KenB :grin: