Can't really remember how I did it (about 5 yrs ago). I might have gone thru the glove box. There's a rubber boot that connects the metal ductwork to the round vents. (I think). Pull it off and stick something down in there to open the flap. I used a thin piece of flat aluminum stock I could bend over the edge of the ductwork to stay in place. Put the rubber boot back on. Plug the vacuum line to the pod. Everything else should work fine.

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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