Please bear in mind....this is how I understand the Automatic Climate Control workings......not sure about the manual version.....but I would assume that the thinking behind the design would be similar.......

there should always be some air flow from the outside.....passing thru the vents in the dash.....and exiting out via the louvers (for lack of a better word) that are located under the chrome/aluminum flashing just aft of the rear doors in the "C" pillar.....

SB

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Diesels:
'85 300D, "Max, Blue Benz", 155K, 27.0 MPG
'84 190D 2.2, "Eva, Brown Benz", 142K, 40.2 MPG
'77 240D (parts car)
'67 Eicher ES 202 Tractor "Otto" (2cyl, Air Cooled, 30HP)
Gassers:
'94 Ford F-150, "Henry", 170K (300 Six) 17.5 MPG
'85 190E 2.3, 148K....Parts Car
'58 Dodge W300M Powerwagon (Flat Fenders) Less than 10 MPG