Mine are rigged open that way. Used a piece of thin aluminum flat stock, bent the end so it would hook over the housing, pulled off the rubber boot that runs from the housing to the vents, stuck the stock down in the vent to hold the flap open then put the rubber boot back on.
Did this about 4 yrs ago so memory is fading. Went in thru the glove compartment I think.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13